verify: convert an exception to bytes before logging

I'm not entirely sure why this code appears to be trying to convert twice, but
it was flagged by pytype:

    File "/mnt/c/Users/Matt/hg/mercurial/verify.py", line 84, in _exc: Function _bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: Exception)
      The following methods aren't implemented on Exception:
      __iter__

Differential Revision: https://phab.mercurial-scm.org/D10181

mercurial/verify.py

@@ -14,6 +14,9 b' from .node import ('
     nullid,
     short,
 )
+from .utils import (
+    stringutil,
+)
 
 from . import (
     error,
@@ -81,7 +84,7 b' class verifier(object):'
 
     def _exc(self, linkrev, msg, inst, filename=None):
         """record exception raised during the verify process"""
-        fmsg = pycompat.bytestr(inst)
+        fmsg = stringutil.forcebytestr(inst)
         if not fmsg:
             fmsg = pycompat.byterepr(inst)
         self._err(linkrev, b"%s: %s" % (msg, fmsg), filename)