Error handling bugfix

Fixes:

• Fixed a bug in error handling triggered when sending process state fails (PM2ZabbixMonitor#processUpdateSendingError).

Usability fix

New features

• Launching the monitor script (pm2-zabbix) without --monitor or --discover now prints a warning message and exits.
• Bonus feature: able to use pm2-zabbix with no options and increased LOG_LEVEL for one-off manual pm2 connectivity checks.

Logging overhaul

New features

• Logging has been rewritten so that it now uses a JSON / line-by-line format thanks to bunyan.
• The tool has been migrated to a zabbix-sender fork so that the same logging format can be used for tracing sender binary execution attempts.