BUG/MEDIUM: snapshot: take the proxy's lock while dumping errors

BUG/MEDIUM: snapshot: take the proxy's lock while dumping errors

The proxy's lock it held while filling the error but not while dumping
it, so it's possible to dereference pointers being replaced, typically
server pointers. The risk is very low and unlikely but not inexistent.

Since "show errors" is rarely used in parallel, let's simply grab the
proxy's lock while dumping. Ideally we should use an R/W lock here but
it will not make any difference.

This patch must be backported to 1.8, but the code is in proto_http.c
there, though mostly similar.