Improve logs when PR-CI is unable to parse the definitions file

[netoarmando]

E.g., when the symlink is pointing to an invalid file this is what PR-CI logs display:

"Nov 28 12:51:02 small-runner-19 bash[937]: INFO: __main__: Skipping PR#3: can't be merged",
"Nov 28 12:51:02 small-runner-19 bash[937]: INFO: __main__: Skipping PR#7: can't be merged",
"Nov 28 12:51:02 small-runner-19 bash[937]: INFO: __main__: Skipping PR#31: can't be merged",
"Nov 28 12:51:02 small-runner-19 bash[937]: INFO: __main__: Skipping PR#67: can't be merged",
"Nov 28 12:51:03 small-runner-19 bash[937]: ERROR: __main__: string indices must be integers"

PR number 82 was the culprit, but there is no way to know that, only to infer based on the previous lines and going to the list of open PRs to see which would be the next after number 67.

Related to #257.