reader/processor: error out for improper rewrite regexp

[anpandey]

It's possible to specify a rewrite regex that validates but doesn't compile such as:

rewrite("(((unmatched-capture-group"|"rewrite)))")

In case we encounter one, exit early instead of letting the server panic.

Do you follow the guidelines?

• [X] I have tested my changes
  • Tested URL rewrite rule rewrite("(((unmatched-capture-group"|"rewrite)))") doesn't crash server
• [X] There is no breaking changes
• [X] I really tested my changes and there is no regression
• [X] Ideally, my commit messages use the same convention as the Go project: https://go.dev/doc/contribute#commit_messages
• [X] I read this document: https://miniflux.app/faq.html#pull-request