Another suggested libgnome API change

[Maciej Stachowiak <mjs noisehavoc org>]

I'd like to add GError arguments to all the gnome-exec functions so
they can be reimplemented in terms of g_spawn and give proper error
reporting (instead of just 0 or -1).

Apps will probably find it easier to port to GError-ized versions of
these calls than g_spawn, but they may as well get decent error
reporting and share implementation in the meantime.

I can get the API-changing part of this done today (just add GError
arguments) and the rest pretty quickly as well.

Thoughts?

 - Maciej