Gracefully fall back to dummy backend

[ghost]

trafficstars

As discussed in https://github.com/servo/servo/pull/24829, we should gracefully fall back to the dummy backend, when the system dependencies are unavailable or fail to work.

/cc @jdm

[Manishearth]

Probably should have a printed warning though

[ghost]

That's a good idea. Is there a central event/log system in servo for that? I suppose trying to render media elements and, when one fails, drawing a placeholder is already a thing in servo, similar to Firefox, right?

[Manishearth]

Oh, just use warn!() or probably error!() from the log crate, we already use it in the codebase, e.g.

https://github.com/servo/media/blob/220ed1388f2ba008b05f5e94aca21dd14aa37290/backends/gstreamer/lib.rs#L162