#23 midi2abc reports errors on standard output

[felix]

midi2abc and other tools in the abcmidi package report errors to the standard output stream. Thus, if the output of midi2abc is redirected to a file or a pipe (instead of using the -o option), any potential error messages are mixed with the actual output, limiting the tool's usefulness.

The preferred location for error messages is, of course, the standard error stream.