rescue: Remove dead code.

This pty code was defended by #if 0 ... #endif and was dead code.
lib: Move common errors to src/errors.c.

This is just code motion.
utils: Fix error messages for external commands that fail (RHBZ#921040).

This adds a common utility function (guestfs___exit_status_to_string)
and a common error function (guestfs___external_command_failed), and
uses them all over the library and tools when converting exit status
in error messages etc.