fail nice on content pack (pre 3.x) import

## Expected Behavior
Give a meaningful feedback on Content pack Import if it can't imported. If it looks like an old content pack, give feedback that this version is not supported anymore.

## Current Behavior
fail with a red short visible banner and no further notice.

## Context
As people do not get the error message or do not understand what the problem might be they get frustrated about Graylog and might move away.

If they just installed the virtual appliance, download a content pack from the marketplace that is labeled with 2 or later and it fails on import this is a bad user experience.