MINOR: proxy: check default proxy compatibility on "add backend"

MINOR: proxy: check default proxy compatibility on "add backend"

This commits completes "add backend" handler with some checks performed
on the specified default proxy instance. These are additional checks
outside of the already existing inheritance rules, specific to dynamic
backends.

For now, a default proxy is considered not compatible if it is not in
mode TCP/HTTP. Also, a default proxy is rejected if it references HTTP
errors. This limitation may be lifted in the future, when HTTP errors
are partiallay reworked.