ao_coreaudio: make conversion error use verbose logging

Using an error level log for something that's not really exceptional was a bit
too much.
ao_coreaudio: skip channel labels marked with and unknown tag

I don't think this is really a very good idea because I think it is
conceptually incorrect but other multimedia software in uses this approach
(VLC and xbmc), and it seems to make the conversion more robust in certain
cases.

For example it has been reported, that configuring a receiver that can output
7.1 to output 5.1, will make CoreAudio report 8 channel descriptions, and the
last 2 descriptions withh be tagged kAudioChannelLabel_Unknown.
ao_coreaudio: cosmetic change of loop ending condition
ao_coreaudio: don't fallback to full waveext

The code was falling back to the full waveext chmap_sel when less than 2
channels were detected. This new code is slightly more correct since it only
fills the chmap_sel with the stereo or mono chmap in the fallback case.
ao_coreaudio: simplify chmap conversion code
ao_coreaudio: handle some unlikely conversion errors

I don't these are likely to happen, but it's better to check for them
since it's not that much code.