This should replace several "segfaults" and "illegal instruction" errors
with messages that contain a bit more context. I also put in a link to
the bug tracker to make users' lives a bit easier.
For context, `__builtin_unreachable`'s behavior is undefined and is
intended only as a mechanism to help the compiler see that code will not
return. I do still include it in the new code because if I don't,
compilation produces a lot more warnings as it can't see that
`NOT_IMPLEMENTED_YET` and `UNREACHABLE` cannot return.
