Now it explicitely stores name of SDL function which caused an error
and generates complete user-readable error message which contains both
function name and SDL error message. Users can now handle SDL2pp
exceptions along with all others in `catch (std::exception&)' and
get complete error info.
While here, fixed incorrect function names in some throw's
Though this is not 100% compatible with SDL2, this makes API much
more consistent and less error prone. For example, you don't need
to store AudioSpec along with AudioDevice just to have your callback
lambda around, you don't need to copy AudioSpec from Wav file just
to fill in the callback (see wav demo), you are no more obliged to
take care of locking AudioDevice while replacing the callback.
