dnsdist: Catch exceptions thrown when handling a TCP response

[rgacogne]

Short description

We need to handle it gracefully if a function called from handleResponse() throws, most likely because the answer received from the backend was not valid.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)