Problem:
========
Fix for bug#11758766, released with 5.6.20, added a new enum
variable "binlogging_impossible_mode".

This breaks the existing naming convention of binlog-related
variables, all starting with "binlog_".

It is also suggested that the variable naming may not be
optimal using the word "impossible".

Fix:
====
A new alias "binlog_error_action" has been added.
Deprecation warnings have been added for
"binlogging_impossible_mode" variable. When ever user sets
or reads the global variable user will get a deprecation
warning.