git-gui: Gracefully display non-aspell version errors to users

git-gui: Gracefully display non-aspell version errors to users

If the user has somehow managed to make us execute ispell instead
of aspell, even though our code is invoking aspell, and ispell is
not recognizing the aspell command line options we use to invoke
it then we don't want a giant usage message back from ispell.

Instead we show the ispell version number, letting the user know
we don't actually support that spell checker.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>