Define a default Monolog logger if inexistant

Define a default Monolog logger if inexistant

In the array 'loggers' the key '@default' is assumed to be present,
hence be sure it is defined: this is the system default for MediaWiki,
it can be overwritten if desired.

This default php://stderr with level ERROR is inspired from Monolog’s
own default, which is php://stderr with level DEBUG: this is similar
but less violent for the average MediaWiki sysadmin.

Bug: T196906
Change-Id: Id74083fc20ddf906c40e6d149095e8ade4c68572