[gnulib-tool-py] func_warning

[address@hidden]

Hi Bruno,

I'm now working with licence code, and I've found that we have to warn user if 
module doesn't have licence. In gnulib-tool we use echo +redirection to stderr, 
so I think we could just use sys.stderr.write(message), but I can create a 
class like GLError if you think that it fits better. What do you think?