Better error handling

[vilhalmer]

Most VRErrors that occur are only exposed to the developer as log messages. Some of the openvr_data methods return a boolean representing success, but these are usually eaten by the XRInterface and therefore the application has no opportunity to react to a failure.

Still being pretty new to Godot, I'm not sure if there are good examples of handling errors out there that would be best to follow, so I'm filing this to get a discussion going. I would at the very least like to see VRErrors bubbled directly up to the XRInterface in some way (maybe like a last_error, so it can be used even by functions that return another type?), but translating them to a more useful form is also fine by me if one exists.