This will allow backends to return a more meaningful error message. Eventually
an error code could be returned by AudioEngine::start and the GUI can then use
AudioBackend::get_error_string to convert the error into a translated error
message directly, or it may be desirable to define its own error messages.
The reasons for not doing that right now is that this is a workable solution
with the least change required.
Tim Mayberry
d85ab8af89