Display post receive warnings from Rails

See merge request gitlab-org/gitlab-shell!288

Add pry gem to allow easy debugging

See merge request gitlab-org/gitlab-shell!289

print_warnings is now using that method in order to print a large
formatted warning message, so renaming the method to be less tied to
the broadcast message functionality.

Warnings are printed using the same method that prints broadcast
messages. This ensures that whatever warnings are returned will be
displayed correctly.

This allows gitlab-shell to be called with an argument of the format
`key-123` or `username-name`.

When called in this way, `gitlab-shell` will call the GitLab internal
API. If the API responds with user information, it will print a
welcome message including the username.

If the API responds with a successful but empty response, gitlab-shell
will print a welcome message for an anonymous user.

If the API response includes an error message in JSON, this message
will be printed to stderr.

If the API call fails, an error message including the status code will
be printed to stderr.

This will enable us to track the human-readable project path with SSH
access in gitlab-shell. Currently the Gitaly logs will only show the
hashed storage and gl_repository path (e.g. project-1234).