It is annoying to see only a single line of error when debugging a chain
of (e.g. `anyhow::Context::context()`-created) errors, and have a
zero exit-code while at it.

Instead Rust supports returning a `Result` type straight from main which
is `Debug`- instead of `Display`-printed, so that - in the case of
`anyhow::Error` - all nested (via `.source()`) `Error`s are printed in
backtrace-like form, and the exit code is appropriately non-zero.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1336>