avoid the default is not a function unclear error (#438)

if there is some issue evaluating a lazy loaded edge function
its default export ends up not being a function and that results
in an unhelpful error like the following:
```
 TypeError: u.default is not a function
```

slightly improve the user experience by catching such error and
letting the user know that something went wrong with the edge
function's evaluation:
```
 Error: An error occurred while evaluating the target edge function (<edge-function-path>)
```

.changeset/brown-eagles-sparkle.md

@@ -0,0 +1,21 @@
+---
+'@cloudflare/next-on-pages': patch
+---
+
+avoid the default is not a function unclear error
+
+if there is some issue evaluating a lazy loaded edge function
+its default export ends up not being a function and that results
+in an unhelpful error like the following:
+
+```
+ TypeError: u.default is not a function
+```
+
+slightly improve the user experience by catching such error and
+letting the user know that something went wrong with the edge
+function's evaluation:
+
+```
+ Error: An error occurred while evaluating the target edge function (<edge-function-path>)
+```

packages/next-on-pages/templates/_worker.js/utils/routing.ts

@@ -91,7 +91,20 @@ export async function runOrFetchBuildOutputItem(
 			case 'function':
 			case 'middleware': {
 				const edgeFunction: EdgeFunction = await import(item.entrypoint);
-				resp = await edgeFunction.default(req, ctx);
+				try {
+					resp = await edgeFunction.default(req, ctx);
+				} catch (e) {
+					const err = e as Error;
+					if (
+						err.name === 'TypeError' &&
+						err.message.endsWith('default is not a function')
+					) {
+						throw new Error(
+							`An error occurred while evaluating the target edge function (${item.entrypoint})`,
+						);
+					}
+					throw e;
+				}
 				break;
 			}
 			case 'override': {