Spring Security Improve StrictHttpFirewall error messaging

Better error strings for invalid header and parameter values.

We have had an outstanding ticket with Cloudflare (https://support.cloudflare.com/hc/en-us/requests/2858014?page=1) for over a month where we have been trying to get to the bottom of what / who is sending an illegal header value. Because StrictHttpFirewall doesn't tell us what the header name is, it's been a guessing game.

PR Work done: Updated exception strings to include the header / parameter name for when the value is invalid.

Realized you might not have access to Cloudflare. Here's a screenshot of the ticket.