Do not throw errors but report diagnostic

[JiaqiZhang-Dev]

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.

Ensure do not throw Error unless we really want the emitter to crash, otherwise we should report error diagnostic.

[RickWinter]

Per @jhendrixMSFT , Perhaps we throw a special exception which is then caught at the top level. This would be reported as an Emitter error rather than a crash. This aligns with typespec's diagnostics model approach.

Additionally, We should be able to emit code models if we complete construction of them. i.e. Don't throw during codegen...