Add a warning when the stoat app is not installed

[tuliren]

When the Stoat app is not installed, the Stoat comment won't show up in the pull requests, which can be confusing to users.

Here is the solution if the Stoat config is enabled, but the Stoat app is not install:

• Add a warning in the log.
• Mark the Stoat step as skipped.

This issue depends on the tracking of Stoat app installation implemented in https://github.com/stoat-dev/stoat/pull/139.