[Minor] Expose Errors history to read-only mode

[Minor] Expose Errors history to read-only mode

The /errors endpoint and its WebUI table were gated behind the
enable (privileged) password. Lower the privilege check so any
authenticated user can view them.

This is safe: the endpoint returns only Rspamd's internal
operational error log (timestamp, pid, level, module, message) —
no email content, PII, or secrets — and read-only users already
see strictly more sensitive data in the History tab (sender/
recipient IPs, scores, symbols). Authentication remains required
and the endpoint is purely informational (no mutation).