Proposal: `subclass` API: use `Result` for functions that can fail

Note: the discussion about this enhancement proposal started here.

In the Rust subclass API, some functions that can fail return a Result. Ex.:

• BaseSrcImpl::create returns Result<gst::Buffer, gst::FlowError>.
• AggregatorImpl::update_src_caps returns Result<gst::Caps, gst::FlowError>.

Other functions retain the bool return value from the C API.Ex.: start, set_caps, …

And other functions return gst::FlowReturn like the C API. Ex.: BaseSinkImpl::render, Aggregator::aggregate, …

As an implementer, functions returning Result feel more convenient as we can use the facilities such as map and map_err and early exit using the ? operator.

This is a proposal to use Result for every functions from the subclass API which can fail. The binding would be in charge of taking care of the required adaptations to conform to the C API. E.g. for BaseSinkImpl::render, this would be there.

Functions returning a boolean could return Result<(), gst::ErrorMessage>. This used to be the case for gst-plugin-simple. In the event of an Err, the binding could also take care of calling gst_element_message_full with the GstGError and optional message from gst::ErrorMessage.

Functions returning a gst::FlowReturn could return Result<gst::FlowSuccess, gst::FlowError>. The binding would take care of returning GST_FLOW_OK in case of an Ok(()) result.

Edit: changed return type to Result<gst::FlowSuccess, gst::FlowError> thanks to @arun's suggestion.