From https://github.com/Seldaek/monolog/blob/master/doc/02-handlers-formatters-processors.md#wrappers--special-handlers:

FingersCrossedHandler: A very interesting wrapper. It takes a logger as parameter and will accumulate log records of all levels until a record exceeds the defined severity level. At which point it delivers all records, including those of lower severity, to the handler it wraps. This means that until an error actually happens you will not see anything in your logs, but when it happens you will have the full information, including debug and info records. This provides you with all the information you need, but only when you need it.

This will be very helpful to actually debug issues and to understand what happened as often an error message in a log standalone doesn't help much. It should be easy to do by just adding it to the configuration. In the future we might also want to add more log entries with debug or trace level (actually not sure if they support trace level).

refs #8616