By passing dbus_g_proxy_call an incorrect signature, we can cause
the function to fail but not provide any error message (if
G_DISABLE_ASSERT is defined).  As smvc pointed out, this can also
become a runtime error when a telepathy CM changes signature, but
the client hasn't been updated.  As such, g_assert isn't an appropriate
action.  See http://bugs.debian.org/541632 for more information.

This patch causes it to set an error when demarshalling a type that it's
not expecting.  Instead of a NULL error, one instead
sees something like the following when the method returns a path
object but the client expected a string:

"modem Create() failed: Expected type gchararray, got type code 'o'"