element: Add API to more easily post messages about flowing issues

element: Add API to more easily post messages about flowing issues

In many parts of the code we raise streaming error when the flow
goes wrong, and each time we create more or less similare error
message. Also that message does not let the application know what
has actually gone wrong. In the new API we add a "flow-return" detail
field inside the GstMessage so that the application has all the information
if it needs it.

API:
    GST_ELEMENT_FLOW_ERROR

https://bugzilla.gnome.org/show_bug.cgi?id=770158