[lldb] Assert that CommandResultObject error messages are not empty

The intention is now that AppendError/SetError/AppendRawError only
be called with some message to show. This enforces that.

For SetError with a Status and a fallback string first assert
that the Status is a failure Status. Then it calls SetError(StringRef)
which checks the message is valid. (which could be the fallback
or the Status')

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D104525