Saving a processing pipeline rule with the same name as an existing rule fails with an empty error message

## Expected Behavior
I'd expect to see an error that tells me when I'm saving a rule that has the same name as an existing rule. Something like "A rule with this name already exists. Please choose another name."

## Current Behavior

I only get this:


## Steps to Reproduce (for bugs)

1. Create a pipeline rule and name it "Rule A"
2. Create a second pipeline rule and also name it "Rule A"
3. Save the second rule. See the non-descriptive error.

## Your Environment

* Graylog Version: Graylog 3.0.0-rc.2+2667ff2