[CHE-198] Create BE Error Handling Docs

docs/ErrorHandling.md

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+# Error Handling
+
+## Quick Links
+
+- [Back End Error Handling](#backend)
+  - [Express Async Errors](#async-errors)
+  - [Custom Error Classes](#custom-errors)
+  - [Global Error Handler](#global-error-handler)
+- [Front End Error Handling](#frontend)
+
+<hr>
+
+<a id="backend"></a>
+
+## Back End Error Handling
+
+<a id="async-errors"></a>
+
+#### express-async-errors
+
+Codehammers makes use of the [express-async-errors](https://www.npmjs.com/package/express-async-errors) package to simplify error handling in asynchronous route handlers and middlewares. This package enables us to simply throw errors instead of manually passing error objects to Express's `next` function. An example would be:
+
+```
+/* with express-async-errors */
+
+const middleware = async (req: Request, res: Response) => {
+  throw new Error('This error will automatically be caught and passed to next')
+}
+```
+
+whereas typically we would have to do this:
+
+```
+/* without express-async-errors */
+
+const middleware = async (req: Request, res: Response, next: NextFunction) => {
+  next({
+    message: 'This error was manually passed to next'
+  })
+}
+```
+
+Using `express-async-errors` eliminates the need for try/catch blocks and allows us to simply throw errors where needed without manually invoking `next`
+
+**How it works**
+The Express framework automatically catches errors thrown in synchronous route handlers and middlewares. However, in the case of asynchronous route handlers and middlewares, Express requires us to manually call `next`, passing in an error object (or anything besides the string `'route'`), in order to invoke the global error handling middleware.
+
+The [express-async-errors package wraps](https://github.com/davidbanham/express-async-errors/blob/master/index.js) Express's Router's [Layer object's](https://github.com/expressjs/express/blob/master/lib/router/layer.js) `handle` property, enabling errors thrown in asynchronous route handlers and middlewares to automatically be caught and passed to `next`. We simply require/import this package once (in `server/app.ts`) and it's functionality is enabled.
+
+A similar approach would require defining our own wrapper such as:
+
+```
+const catchAsync = (fn) => {
+  return (req, res, next) => {
+    fn(req, res, next).catch(next)
+  }
+}
+```
+
+we would then have to wrap every route handler and middleware in this catchAsync function to automatically catch errors and pass them to next:
+
+```
+const middleware = catchAsync(async (req, res) => {
+  throw new Error('Oops, something went wrong')
+})
+```
+
+Much nicer not having to manually wrap every route handler and middleware!
+
+<a id="custom-errors"></a>
+
+#### Custom Error Classes
+
+Codehammers uses custom error classes for handling the most common types of errors that occur on the server. These classes can be found in the `server/errors/` directory. These error classes and their corresponding status codes are:
+
+- `BadRequestError` : 400 (requires a `message` argument when instantiated)
+- `DatabaseConnectionError` : 500
+- `InternalError` : 500
+- `NotAuthorizedError` : 401
+- `NotFoundError` : 404
+- `RequestValidationError` : 400 (requires an array of `ValidationError`s as an argument when instantiated - see [below](#request-validation-error))
+
+Each of these error classes extend our `CustomError` abstract class (which itself extends JavaScript's built-in `Error` object). This abstract class cannot be instantiated, but is used to enforce a structure for our custom error classes: requiring a `statusCode` property, a `message` property, as well as a `serializeErrors` method. The `serializeErrors` method ensures that errors sent back to the client are consistently formatted as an array of objects, each object containing a `message` property and an optional `field` property - used for validation errors.
+
+<a href="request-validation-error"></a>
+
+##### RequestValidationError
+
+Another custom utility error class is `ValidationError`. This error is for user inputs that fail validation, such as an invalid email, or a password that is too short.
+
+When validating multiple user inputs, we create an array for validation errors and push in a `ValidationError` for each failed validation. We then throw a `RequestValidationError` if this array contains any `ValidationError`s. An example of this could be:
+
+```
+const createUser = async (req: Request, res: Response) => {
+  const { email, password } = req.body
+
+  const validationErrors: ValidationError[] = []
+  if (!email) {
+    validationErrors.push(new ValidationError('Please provide a valid email', 'email'))
+  }
+  if (!password) {
+    validationErrors.push(new ValidationError('Please provide a valid password', 'password'))
+  }
+  if (validationErrors.length) {
+    throw new RequestValidationError(validationErrors)
+  }
+
+  // continue if input validation checks pass...
+}
+```
+
+`ValidationError`s include a relevant `message` for the user, and the `field` representing which input failed validation, passed as arguments when instantiating a `ValidationError`.
+
+Having the ability to collect and send back multiple errors for each validation failure helps our front end developers relay useful information to the user about which inputs were invalid and need fixing.
+
+<a href="global-error-handler"></a>
+
+#### Global Error Handler
+
+We are currently migrating our error handling to make use of `express-async-errors` and our custom error classes. While this migration is ongoing, we still need to handle error objects that have been manually passed to Express's `next` function. Our global error handler (`server/middleware/errorHandler.ts`) is setup to handle both of these cases, while still sending back consistently formatted errors. If an error is neither a custom error class or manually caught error object, we send back a generic `InternalError` with `500` status code.
+
+<hr>
+
+<a id="frontend"></a>
+
+## Front End Error Handling
+
+TBD