Provide LCB_LOG_ERR define

This is useful for LOG* macros which accept a level base as a parameter.
Unfortunately 'ERROR' is a reserved word on Win32, thus messing up the
macro concatenation.

Change-Id: I958cf86def412bfd4588867d4e48469663be542d
Reviewed-on: http://review.couchbase.org/31818
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>