🐛 Bug Report/Feature Request: improve fallback service error message on js code compilation errors

[dario-piotrowicz]

The module fallback service given code that contains a syntax error returns an error response saying that the requested module could not be found, instead of clearly saying that a compilation error occurred, while using the service I encountered this causing me confusion and making me question if my code was failing to correctly fetch the module, so I think it would be great if this could be improved.

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minimal reproduction made by using miniflare

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I think that the main issue is the fact that here on errors we simply only return a failed-to-fetch-module error:

workerd/src/workerd/server/server.c++

Lines 2872 to 2876 in 1a99c34

	} catch (...) {
	auto exception = kj::getCaughtExceptionAsKj();
	KJ_LOG(ERROR, "Fallback service failed to fetch module", exception, spec);
	return kj::none;
	}

I'm guessing that in order to catch syntax errors (or any possible errors?) and surface them correctly to the user we should try-catch this piece of code?

workerd/src/workerd/server/server.c++

Line 2866 in 1a99c34

auto module = WorkerdApi::tryCompileModule(js, moduleBuilder, observer, featureFlags);