Merge: Revamp app.nit print_error and print_warning

Should we move `print_error` to the standard library? It could be useful and specialized for many usages. Android redirects to logcat, PNaCl could redirect to the JavaScript console, and nitcorn could tee to console and a file.

All the current errors printed on `sys.stderr` are lost on Android. Redirecting the stream is not ideal as each call to `write` would use a new line on the Android log.

Pull-Request: #1329
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>