convert: print exception message directly
Otherwise the default repr() implementation will b'' prefix the
returned str on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D5729
crecord: always return a str from uihunk.__repr__
Otherwise Python 3 complains about it returning bytes.
Differential Revision: https://phab.mercurial-scm.org/D5728