error message should tell user to run audiotools-config when filename supplied but not supported

[macmenot]

Currently if (e.g.,) you haven't installed faac/faad to enable support for m4a files and you attempt to interact with them, you get a generic error message that doesn't make it immediately obvious how you would resolve the problem:

% trackrename "--format=%(track_number)2.2d-%(track_name)s.%(suffix)s" *.m4a
*** Error: you must specify at least 1 supported audio file

When the user has supplied at least one filename, but it is an unsupported format (due to missing binaries etc.) it would be helpful if this error message pointed the user at the audiotools-config command as a way of seeing what binaries they currently have and what they need to install to enable operation on those files.