Content Pack with Parameter does not raise error if parameter is left empty during install

## Expected Behavior
When installing a content pack with parameters without defaults the form should show an error
if the parameter is left empty.

## Current Behavior
Right now the modal closes without any feedback to the user.

## Possible Solution
Show an error when the parameter field is left empty.

## Your Environment
* Graylog Version: 3.3 development (might be in a former version)