notify about parsing errors in response headers and body

[Emmanuel Bosquet]

Sōzu returns for now:

• standard HTTP responses with HTTP status codes (301, 502, etc)
• custom 404 and 503 responses (custom HTML strings)

It would be beneficial for the end user to receive additional context when receiving an HTTP status code. Trickling Kawa parsing errors in a response header or in the custom HTML strings would allow a client to know that their request is wrongly formed, or that their backend creates wrong responses (for instance : an error about redundant content-length headers of the backend would be forwarded to the client in a 502 answer).

[Emmanuel Bosquet]

This is done since #1080 !