pad: Handle sticky event errors

pad: Handle sticky event errors

Use GstFlowReturn to internally pass events between pads.
When we sticky events cause an error, translate this error into a GstFlowReturn.
Caps events will, for example, generate a NOT_NEGOTIATED return when the event
function returns an error.
This allows us then to refuse sending buffers if one of the sticky events is
refused and generate a correct error return value.