gst/gstpad.*: Added _CUSTOM error and success GstFlowReturn that can be used be eleme...

gst/gstpad.*: Added _CUSTOM error and success GstFlowReturn that can be used be elements internally.

Original commit message from CVS:
* gst/gstpad.c: (gst_flow_get_name), (gst_flow_to_quark):
* gst/gstpad.h:
Added _CUSTOM error and success GstFlowReturn that can be
used be elements internally.
Added macro to check for SUCCESS flowreturns.
API: GST_FLOW_CUSTOM_SUCCESS
API: GST_FLOW_CUSTOM_ERROR
API: GST_FLOW_IS_SUCCESS
* tests/check/gst/gstpad.c: (GST_START_TEST), (gst_pad_suite):
Added check for GstFlowReturn sanity.