Redirect error messages to current standard error stream

[tonyta]

We had a script that ran a Rails migration. Since its output is noisy, we swallowed stdout. The exit code was 1 but there was no message because annotate_models puts its error messages to stdout instead of stderr.

This PR redirects error messages to the current standard error stream $stderr.

Before

$ bundle exec rake annotate_models >/dev/null
$ echo $?
1

After

$ bundle exec rake annotate_models >/dev/null
No models found in directory 'app/models'.
Either specify models on the command line, or use the --model-dir option.
Call 'annotate --help' for more info.
$ echo $?
1

[ctran]

Thanks!