onsub extension not runnable with hg 4.2.2
Created originally on Bitbucket by simohe (S. Heimberg)
This is partially fixed in this repo: https://bitbucket.org/wizz/onsub/
But the variable errmsg is used before assigned in line
errmsg = '%s: %s' % (errprefix, errmsg), see issue 1 in wizz/onsub.
By the way, there is no clear maintainer for this extension, see comment on https://www.mercurial-scm.org/wiki/OnsubExtension