Improve error message when exception happens

[ThomasdOtreppe]

System info:

• Ubuntu 14.04.3 64 bit
• Logstash 2.1.0 from Elastic deb repo

Due to date parsing issue, I got the following in my logs:

{:timestamp=>"2015-11-25T13:50:28.229000-0700", :message=>"Ruby exception occurred: can't dup NilClass", :level=>:error}

After debugging, I figured that it came from the Ruby filter because parsing failed (and the field I was working on wasn't set.

It would be nice if the error indicated it came from the ruby filter, that would help debugging a lot.