Missing error processing in ControllerProxy

[elgris]

How to reproduce

1. Install whole ZMON stack
2. Break ZMON Controller API so it returns errors (e.g. because of broken auth configuration, so requests from data-service are not authorised)
3. Observe Index Out of Bounds Exception coming from de.zalando.zmon.dataservice.proxies.ControllerProxy#proxyForLastModifiedHeader

Explanation

When ZMON Controller API returns error responses, data-service still tries to get "Last-Modified" header from there and fails. Need to make sure that data-service tries to check if the header in the response before it tries to extract it.

Acceptance criteria

• Instead of Out of Bounds Exception data-service must report that "Last-Modified" header was not found and provide response data, so it's easier for developers to investigate