configtest misses Invalid conversion type in mutate:convert

version logstash 1.5.3, installed from the official repo (apt)

**description**
An invalid field type in a mutate conversion doesn't raise any flags when run with --configtest. When it errors out, it does suggest you use configtest though!

**to reproduce**
- add the following to your config:
- filter { mutate { convert => {'some_field' => 'configtest_cant_see_me' } } }
- run logstash configtest
- /opt/logstash/bin/logstash -f /etc/logstash/conf.d/ --configtest
- run logstash for real
- /opt/logstash/bin/logstash -f /etc/logstash/conf.d

**output from command line configtest**
/opt/logstash/bin/logstash -f /etc/logstash/conf.d/ --configtest
'[DEPRECATED] use `require 'concurrent'` instead of `require 'concurrent_ruby'`
Configuration OK

**output in logstash.stdout**
/opt/logstash/bin/logstash -f /etc/logstash/conf.d
'[DEPRECATED] use `require 'concurrent'` instead of `require 'concurrent_ruby'`
Error: Cannot register filter mutate plugin. The error reported is:
Invalid conversion type 'configtest_cant_see_me', expected one of 'string,integer,float,boolean'
You may be interested in the '--configtest' flag which you can
use to validate logstash's configuration before you choose
to restart a running system.