transaction: explain why some recovery failed

Right now we issue a message about failing to recover some file, but not why.
It seems useful to add some information about that.

(This is an opportunity improvement while looking at something next to that.)

Differential Revision: https://phab.mercurial-scm.org/D10842

mercurial/transaction.py

@@ -94,8 +94,9 b' def _playback('
                 try:
                     util.copyfile(backuppath, filepath, checkambig=checkambig)
                     backupfiles.append(b)
-                except IOError:
-                    report(_(b"failed to recover %s\n") % f)
+                except IOError as exc:
+                    e_msg = stringutil.forcebytestr(exc)
+                    report(_(b"failed to recover %s (%s)\n") % (f, e_msg))
             else:
                 target = f or b
                 try: