[python] Emitters should not crash on purpose

[msyyc]

See from #5660 (comment)
Emitters should report errors as many as possible therefore if possible, emitters should not crash on purpose (for example throw new Error).
Throwing an error will cause the emitter to crash, and when the emitter crashes, typespec compiler will flag this as a bug in the emitter and recommend the user to open an issue for it.
For those unexpected, it is fine that the emitter would crash, because it is "unexpected", we could never prevent it therefore it should be categorized as a bug.
Please go through the code to ensure we do not throw Error unless we really want the emitter to crash, otherwise we should report error diagnostic.