evolve: improve error messages when conflicts occur This patch improves the error messages when conflicts occur. First, we drop the line 'evolution failed', that is not the best line we can show and evolution didn't failed, it's just interrupted by the conflicts and when user will run `hg evolve --continue`, things will be fine. I still remember when I first saw 'evolution failed', I got a bit scare as am I in a recoverable position or not. So let's drop this scary line. Second, we replace the error messages to say `resolve conflicts and see help-topic`. The help topic was added recently and documents all the three flags very well. Addition of tests also showed that all the three flags works fine with all the three instability type. So we should advertise them more. Third, we now raise the error with our error message rather than raising MergeFailure and having evolution related text in hint or stderr above. This increase the focus on the error message we want to show. After this patch, I think error messages by evolve in case of conflicts will be same in every case.