Add enums to AudioBackend class for getting standard error and device name strings

Add enums to AudioBackend class for getting standard error and device name strings

These could also be used for return values but are initially intended for
shared translations of error messages between backends. Ideally IMO translation
should occur in the GUI(as there may be more than one) in response to these(or
similar) error codes but that would take agreement and a fair bit of
refactoring. As it is there are errors that occur in the backends that users
will want to know about this at least allows consistent translations to be
shared between backends.