[TEST] better failure message when back location is missing
MultiGet & MultiTermVector api: fail when using no routing and an alias to an index that has routing required (for that doc type)

Made sure that the routing required check is performed against the concrete index, added use of aliases to existing routing tests.

Taken the change to unify the failure message as well to this form: routing is required for [" + index + "]/[" + type + "]/[" + id + "]