liburbi: display errors.
liburbi: display errors.

Currently the interface (which does not throw at all) makes it too easy to
hide errors: we return messages, which can be error messages, but we can
easily read their value, even if there is none!

Something safer is needed.

	* src/liburbi/usyncclient.cc (waitForTag): Show errors on stderr,
	just in case.

urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi