Unhelpful error message when trying to install content pack with failing version constraint

When trying to install a content pack that requires a graylog version higher than that of the server, users get a very generic error message in the frontend that doesn't state why the install failed.

## Expected Behavior
The error message should state that the import failed, because the server has version X, but the content pack requires at least version Y.

## Current Behavior
A generic error message that states that the install failed with a status code of 500.

## Possible Solution
-

## Steps to Reproduce (for bugs)
See below. Really easy in dev environment.

## Context
Noticed this while testing content packs on my dev environment which has a lot of failing version constraints by default, because the export content packs always require >=3.2.0 while the server only offers 3.2.0-SNAPSHOT

## Your Environment
dev environment

* Graylog Version: 3.2.0-SNAPSHOT
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: