py3: convert an exception message to bytes

One of our users ran into this (unrelated to my recent work on
more specific errors).

Differential Revision: https://phab.mercurial-scm.org/D9184

mercurial/merge.py

@@ -1309,7 +1309,8 b' def batchremove(repo, wctx, actions):'
             wctx[f].remove(ignoremissing=True)
         except OSError as inst:
             repo.ui.warn(
-                _(b"update failed to remove %s: %s!\n") % (f, inst.strerror)
+                _(b"update failed to remove %s: %s!\n")
+                % (f, pycompat.bytestr(inst.strerror))
             )
         if i == 100:
             yield i, f