Gracefully fall back to dummy backend

[ghost]

As discussed in servo/servo#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.

media/backends/gstreamer/lib.rs

Line 162 in 220ed13

warn!("Trying to exec media action on an unknown client context");