rebase: improve error message on improper phases The previous error message had two issues: The first issue was that it wasn't, in fact, an error but a warning, even though it described a fatal error condition preventing the successful completion of the command. The second was that it didn't mention the immutable changesets, leaving the user guessing at the true cause of the error. The main downside to this change is that we now get an 'abort: can't abort...' message which technically contradicts itself. In this case, I blame that on the two uses we have for the word; if it weren't for backwards compatibility, we could make util.Abort print out 'error: <whatever>'.