patch: fix a str + bytes issue in an exception handler stable

patch: fix a str + bytes issue in an exception handler

Flagged by pytype.

Differential Revision: https://phab.mercurial-scm.org/D7466

--- a/mercurial/patch.py	Wed Nov 20 08:11:21 2019 -0800
+++ b/mercurial/patch.py	Thu Nov 21 14:28:28 2019 -0500
@@ -2290,7 +2290,7 @@
             try:
                 current_file = patcher(ui, gp, backend, store, eolmode=eolmode)
             except PatchError as inst:
-                ui.warn(str(inst) + b'\n')
+                ui.warn(stringutil.forcebytestr(inst) + b'\n')
                 current_file = None
                 rejects += 1
                 continue