[views] fix possible UnboundLocalError in ErrorView
This fixes:
File "/usr/local/lib/python3.7/dist-packages/cubicweb/web/views/management.py", line 156, in call binfo = text_error_description(ex, excinfo, req, eversion, cversions) UnboundLocalError: local variable 'eversion' referenced before assignment
This occur when self._cw.cnx is False and submit-mail
is configured.
Closes #223 (closed)