Handle non-existent APIs gracefully

[lgarron]

This expands on #13.

Right now, we e.g. call navigator.storage.persist() regardless of whether navigator.storage exists (for almost everyone, it doesn't exist right now). We should surface the absence of an API better.

[KisaragiEffective]

Old issue, but I would like have this feature. Firefox does not provide some APIs that Chrome provides, such as MIDI and USB.