Improve error handling in arangorestore.

In particular:
  - Notice if collections mentioned in _from and _to fields of edges
    do not exist.
  - In cluster case make the coordinator report errors from the
    DBservers better.
  - Fail more quickly if arangorestore does not go well.
Merge branch 'devel' of ssh://github.com/triAGENS/ArangoDB into devel