Pass notification of failure of watches onto the caller.

Pass notification of failure of watches onto the caller.

This is particularly useful for situations where the user might really want to
be notified about a failure, for instance, in a backup application.

Empty paths are not treated as an error in calling, as the user code cannot
really do anything sensible to handle this error, but empty paths should not be
used.

Change-Id: Iddb44fd39f4e3fac5c3f9f60fb7999e1833280a8
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Denis Dzyubenko <denis.dzyubenko@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>