[MEDIUM] implement error dump on unix socket with "show errors"

[MEDIUM] implement error dump on unix socket with "show errors"

The new "show errors" command sent on a unix socket will dump
all captured request and response errors for all proxies. It is
also possible to bound the log to frontends and backends whose
ID is passed as an optional parameter.

The output provides information about frontend, backend, server,
session ID, source address, error type, and error position along
with a complete dump of the request or response which has caused
the error.

If a new error scratches the one currently being reported, then
the dump is aborted with a warning message, and processing goes
on to next error.