py3: fix stringmatcher() to byte-stringify exception message

Spotted while writing regexp variant of stringmatcher().

mercurial/utils/stringutil.py

@@ -361,7 +361,9 b' def stringmatcher(pattern, casesensitive'
                 flags = remod.I
             regex = remod.compile(pattern, flags)
         except remod.error as e:
-            raise error.ParseError(_(b'invalid regular expression: %s') % e)
+            raise error.ParseError(
+                _(b'invalid regular expression: %s') % forcebytestr(e)
+            )
         return kind, pattern, regex.search
     elif kind == b'literal':
         if casesensitive:

tests/test-revset.t

@@ -1448,6 +1448,9 b' test author'
     (string '('))
   hg: parse error: invalid match pattern: (unbalanced parenthesis|missing \),.*) (re)
   [255]
+  $ log 'desc("re:(")'
+  hg: parse error: invalid regular expression: (unbalanced parenthesis|missing \),.*) (re)
+  [255]
   $ try 'grep("\bissue\d+")'
   (func
     (symbol 'grep')