If we don't handle the case of RTSPServer::attach() failing, we end up
with a panic. Unfortunately, we don't get any details from the
underlying call so we've to live with a generic error. :(
The 'failure' crate has been stale for quite some time and better
alternatives has been developed since its introduction. We choose the
'anyhow' and 'derive_more' to replace it.
This code rework the examples to use the new 2018 edition and also
rework the code to avoid using unnecessary 'extern crate' calls.
The 'use extern crate gstreamer as gst', as well as the other gstramer
related crates, were kept, otherwise we'd need to do it on 'Cargo.toml'
but it would make it more difficult to figure out the respective crate
name.
