Improve data node directory compatibility check error response

## Expected Behavior
All types of errors occuring during the directory compatibility check in the data node migration wizard should bubble up to the UI.

## Current Behavior
Currently, only `IncompatibleIndexVersionException` and `ValidationException` are wrapped into a parsable response. All others return a 500 message, causing JSON parsing to fail in Graylog.

## Possible Solution
Handle all exceptions

## Steps to Reproduce (for bugs)

1. Find a way to throw a exception other than the two above in `indices-directory/compatibility`
2. Try to run compatibility check in migration wizard

## Context
found during testing by https://community.graylog.org/t/be-a-part-of-graylog-development-call-for-testers/33125/4

## Your Environment

* Graylog Version: 6.1.0-alpha.6