Better handling of failed date filter parsing

The date filter needs better failure behavior. One suggestion is to include the event which failed to be parsed. The error message could be more friendly too (instead of an exception/stack trace.)

As noted in LOGSTASH-784 Resolved ... Date filter, when failing, should show the input text and the config

Is it possible to log the complete event in lib/logstash/filters/date.rb on line 213?

213           @logger.warn("Failed parsing date from field", :field => field,
214                        :value => value, :exception => e,
215                        :backtrace => e.backtrace)

Sometimes it's not possible to determine which input logs with a malformed timestamp.