Rename error_message to warning
Rename error_message to warning

Since we're not actually raising an error, just outputting a warning
message to $stderr.

NeimadTL/ruby,Insti/xruby,Insti/xruby,Insti/exercism-ruby,Insti/exercism-ruby,Insti/exercism-ruby,NeimadTL/ruby,NeimadTL/ruby,exercism/xruby,exercism/xruby