Improve error reporting when BOSH env vars are missing
Improve error reporting when BOSH env vars are missing

To access the webdav blobstore we use 'bosh ssh' and require BOSH env vars to be present. Let's be clearer when they are missing.

[#156533708]

cloudfoundry-incubator/bits-service-release,cloudfoundry-incubator/bits-service-release,cloudfoundry-incubator/bits-service-release,cloudfoundry-incubator/bits-service-release