Review all FatalError throws

I've seen a few code snippets like this:

    // todo: emit a diagnostic messsage
    throw new FatalError();

This is problematic for two reasons:

1. User doesn't get any useful feedback.
2. Worse, on the command-line it will show up as an internal error, because there must not be a FatalError without a CompilerDiagnostic thingy having been issued.

Concrete work items:

1. Review all places in the code where a FatalError is thrown, and make sure that a corresponding CompilerDiagnostic thingy was issued before.
2. Rethink best practices, as it is too easy to create mismatches between FatalError and CompilerDiagnostic (how did this get past code review?).