errors: make OutOfBandError extend Abort
I'm about to create a new `RemoteError` exception and make
`OutOfBandError` extend it. This patch prepares for that.
Differential Revision: https://phab.mercurial-scm.org/D10465
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif