Renamed RtError class to RtAudioError and embedded it in RtAudio.h.

Renamed RtError class to RtAudioError and embedded it in RtAudio.h.

Deleted RtError.h from distribution and renamed all references to
RtError in the documentation, test files, and Windows .dsp files.
The version number was incremented to 4.1.0 in anticipation of
the next release, as this change affects the API.