py3: fix curses chunkselector fallback (when diffs are too large) on py3

Previously we showed the message using Exception.message, which is removed in
py3. Since crecordmod.fallbackerror inherits from error.Abort, we can just use
`b'%s' % exception` to print the message. This does not print the hint, but
that's fine - we don't set one. We inherit from error.Abort so that if a
codepath doesn't handle fallback specially, it exits to the terminal with a sane
message instead of an unknown exception error.

Differential Revision: https://phab.mercurial-scm.org/D7912

mercurial/cmdutil.py

@@ -386,7 +386,7 b' def filterchunks(ui, originalhunks, usec'
                 ui, originalhunks, recordfn, operation
             )
     except crecordmod.fallbackerror as e:
-        ui.warn(b'%s\n' % e.message)  # pytype: disable=attribute-error
+        ui.warn(b'%s\n' % e)
         ui.warn(_(b'falling back to text mode\n'))
 
     return patch.filterpatch(ui, originalhunks, match, operation)