MEDIUM: log-format: strict parsing and enable fail

MEDIUM: log-format: strict parsing and enable fail

Until now, the function parse_logformat_string() never fails. It
send warnings when it parses bad format, and returns expression in
best effort.

This patch replaces warnings by alert and returns a fail code.

Maybe the warning mode is designed for a compatibility with old
configuration versions. If it is the case, now this compatibility
is broken.

[wt: no, the reason is that an alert must cause a startup failure,
 but this will be OK with next patch]