errors: Even clearer error message.

Obviously no one reads the FAQ, so make the error message
even clearer.

The new error messages are variation on this:

libguestfs: error: /bin/false exited with error status 1.
To see full error messages you may need to enable debugging.
Do:
  export LIBGUESTFS_DEBUG=1 LIBGUESTFS_TRACE=1
and run the command again.  For further information, read:
  http://libguestfs.org/guestfs-faq.1.html#debugging-libguestfs
You can also run 'libguestfs-test-tool' and post the *complete* output
into a bug report or message to the libguestfs mailing list.
fish: c-pointer: Print the pointer in hexadecimal.

$ guestfish
><fs> c-pointer
0x1fb4f80

This updates commit d8c43d556b45f4c9676fdbfce0e9fe07b709635a.