logstash: Update regex in 'exception' grokker

Follows-up 399ba2fecf in mediawiki-core which changes the message
from "Exception from line ..." to "{ClassName} from line ...".

Change-Id: I895e8dba1d532a55c0730598c13d8e9c2048ced9
Merge "logstash: Update regex in 'exception' grokker" into production