MEDIUM: log-format: Use standard HAProxy log system to report errors

MEDIUM: log-format: Use standard HAProxy log system to report errors

The function log format emit its own error message using Alert(). This
patch replaces this behavior and uses the standard HAProxy error system
(with memprintf).

The benefits are:
 - cleaning the log system

 - the logformat can ignore the caller (actually the caller must set
   a flag designing the caller function).

 - Make the usage of the logformat function easy for future components.