indicate mutliple PRs found

Hello @juliangruber ,
Found this action and I like it, would like to use in my GHA automation flows, makes a nice combo with `peter-evans/close-pr`.
One thing that I would need is a clear indication of multiple matches, which is not here at the moment.
I could see following options:
* fail in case of multiple matching items with a proper and sufficient error message
* revamp outputs to allow returning multiple matches

While second option would be a breaking change (I'd prefer not returning `{ pr_info | pr_info[] }`, but always return `pr_info[]` and process its length on my workflow, first one seems low-hanging fruit and fully backwards-compatible.

Would you agree with such change (first option) and accept corresponding PR?

Best regards,

Start at the action's current pull-request matching and output behavior, then determine how multiple matches are represented. The issue favors a backwards-compatible failure with a clear error, but also discusses returning an array, so confirm the intended option before implementation. Done means multiple matches are clearly indicated without changing the existing single-match behavior.

Written by the indexing model from the issue text.