grep: convert an exception to bytes for a warning message

Caught by pytype:

    File "/mnt/c/Users/Matt/hg/mercurial/commands.py", line 3457, in grep: Function _bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: re.error)
      The following methods aren't implemented on re.error:
      __iter__

Differential Revision: https://phab.mercurial-scm.org/D10173

mercurial/commands.py

@@ -3444,7 +3444,8 def grep(ui, repo, pattern, *pats, **opt
         regexp = util.re.compile(pattern, reflags)
     except re.error as inst:
         ui.warn(
-            _(b"grep: invalid match pattern: %s\n") % pycompat.bytestr(inst)
+            _(b"grep: invalid match pattern: %s\n")
+            % stringutil.forcebytestr(inst)
         )
         return 1
     sep, eol = b':', b'\n'