[lldb] Remove asserts in CommandReturnObject SetError and AppendError
I added asserts to these in https://reviews.llvm.org/D104525.
They are available (directly or otherwise) via the API so we
should not assert.
Restore the previous behaviour. If the message
is empty, we return early before printing anything.
For SetError don't assert that the error is a failure.
The remaining assert is in AppendRawError which
is not part of the API.
Reviewed By: teemperor
Differential Revision: https://reviews.llvm.org/D104778