wireproto: introduce pusherr() to deal with "unsynced changes" error
The behaviour between http and ssh still differ:
- the "unsynced changes" is seen as a remote output in the http cases
- but it is correctly seen as a push error for ssh
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case