Currently the last backend error string is only set when calling
AudioBackend::start. Errors that occur when calling other AudioBackend methods
like AudioBackend::set_buffer_size do not set the last backend error string.
So until all the required AudioBackend methods return an ErrorCode and
last_backend_error() can be removed just set it to a default string.
Until all errors that occur are correctly indicated by returning an error code
there will still be situations where last_backend_error() is not indicative of
the true error. For instance AudioEngine::start is called and it fails but
returns a valid error code so last_backend_error() is set, then some other
engine/backend method is called that fails and as last_backend_error is not
set the GUI will display the incorrect error message.
Tim Mayberry
3eb4b43fa6