Handle interrupted AJAX calls gracefully

[roytanck]

Currently, if the AJAX call that Plugin Report does to retrieve a plugin's info fails completely, the process breaks. All following plugins are not processed, and thus no info is shown for them.

We should add at least a very basic callback for when this happens, that displays a row, probably with an error message. Following rows should be processed.

This is related to https://wordpress.org/support/topic/wordpress-6-7-1-compatible/ .

[apermo]

PR at #95.

The AJAX call had no error handler — if a request failed (network, timeout, 500), the success callback never fired and rtpr_process_next_plugin() was never called, breaking the chain.

Fix adds:

• .fail() handler that shows an error row and continues to the next plugin
• try/catch around JSON.parse for malformed responses

This complements #92 which prevents the server-side from failing — this PR handles transport-level failures.