Handle deletion of source files

[simonhorlick]

Currently this causes things to break. For example:

(node:53366) UnhandledPromiseRejectionWarning: Error: ENOENT: no such file or directory, open '/Users/simon/src/github.com/moonad/Kind/base/Fmt/motive2.kind'
(node:53366) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:53366) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
TypeError: Cannot read property 'positionAt' of undefined
    at /Users/simon/src/github.com/simonhorlick/formality-vscode/server/out/fm.js:30:33
    at Array.map (<anonymous>)
    at lspResponseToDiagnostics (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/out/fm.js:27:16)
    at Object.computeDiagnostics (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/out/fm.js:61:29)
    at /Users/simon/src/github.com/simonhorlick/formality-vscode/server/out/server.js:104:23
    at CallbackList.invoke (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/node_modules/vscode-jsonrpc/lib/events.js:62:39)
    at Emitter.fire (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/node_modules/vscode-jsonrpc/lib/events.js:121:36)
    at /Users/simon/src/github.com/simonhorlick/formality-vscode/server/node_modules/vscode-languageserver/lib/main.js:171:38
    at handleNotification (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/node_modules/vscode-jsonrpc/lib/main.js:502:43)
    at processMessageQueue (/Users/simon/src/github.com/simonhorlick/formality-vscode/server/node_modules/vscode-jsonrpc/lib/main.js:273:17)