Provide better error messages in TS compiler

[bartlomieju]

@bartlomieju it is compiling .js without checkJs on that is the problem. So there is actually nothing to emit, which isn't actually unexpected. Clearly we shouldn't panic, but I am not 100% sure what the right response in this situation is. Maybe a throw in JS land? Something like:

Error: The compilation did not emit anything.  Are you sure your configuration is valid?

Also, maybe we turn checkJs: true by default on the compiler APIs?

@sdhoward in your particular case, you aren't telling the compiler to actually check JavaScript. I suspect if you did set the compiler option to checkJs: true you would get a more meaningful error as a work around for now.

Originally posted by @kitsonk in #6747 (comment)

There are a few asserts in TS compiler that do not have meaningful error messages. We should update each assert with elaborate description suggesting what might be the problem.

[bartlomieju]

Not relevant in light of #7225