Readme plugin - option to ignore/hide 500 errors

### Describe the feature

Checking a user in backstage using githubOrg plugin to load users, the plugin seems to add a source location of `backstage.io/source-location: url:https://github.com/`, which causes the readme plugin to load `https://github.com/Readme.md` which results in a 500 error on the user/group page in backstage.

Would be good to have an option to opt into (or opt out of) loading the plugin based on entity types (users/groups)
and/or be able to ignore 500 errors from the plugin.

I was thinking teh `hideIfNotFound` option might work but it does not seem to, maybe since it is not a 404 returned.

**Explain why this enhancement would be useful** to most users. You may also want to point out the other projects that solved it better and which could serve as inspiration.

This would be better UI in backstage and if we opt in/out of certprobably avoid attempts to load a readme on resources that they would have them.

### Additional context

Start by tracing the Readme plugin's loading behavior for Backstage users and groups, including how the existing hideIfNotFound option handles the 500 response. Decide whether entity-type filtering, ignoring 500 errors, or both is in scope, then verify that affected user and group pages no longer display the error while valid README loading still works.

Written by the indexing model from the issue text.