eAccelerator doesn't like anonymous functions.

- Change to private instance method for error handler.
Changes from comments.

- Move private instance variable.
- Reset errorMessage.
- See https://github.com/Seldaek/monolog/pull/248
Merge branch '1.6.0_with_fix_for_eaccelerator'

Conflicts:
	src/Monolog/Handler/StreamHandler.php