Add a Customize_utils module. This contains common error/warning/info
functions, and also quote = Filename.quote.
Examine every existing call to printf/eprintf and change where
necessary so that:
- error is used instead of eprintf + exit 1
- warning no longer needs ~prog argument (it is added by Utils module)
- any verbose output should go to stdout, not stderr
- info is used to print general informational messages
Also, don't pass ~prog parameter around. Instead we just get it from
the executable name.