Review failure messages of built-in listeners

1. Look for "Verify the consistency" and make it start with "Maybe".
2. Improve the "which has a sourceVariableName variable" part. It's confusing.

For context/motivation see:

https://github.com/kiegroup/optaplanner/pull/1751#discussion_r778738804

We typically start the advise line after the \n with "Maybe", so "Maybe verify the consistency of ...".