dlnasrc: gst_dlna_src_uri_set_uri: set a GError when failing

GStreamer expects that a gst_uri_handler_set_uri() implementation returning
FALSE also set the GError with the details about the failure. By not doing so,
gst_element_make_from_uri() was crashing when trying to deference a NULL
pointer.
Merge pull request #131 from gdesmott/uri-crash

dlnasrc: gst_dlna_src_uri_set_uri: set a GError when failing
dlnasrc: remove is_uri_initialized

It's not needed any more as dlna_src_uri_init() is called only by
dlna_src_uri_assign().
Furthermore it was bugged as dlna_src_uri_assign() should have to set it back to
FALSE.