explain that a requirement caused the error, otherwise you search until you die

Signed-off-by: Nico Schottelius <nico@bento.schottelius.org>

cdist/emulator.py

@@ -194,7 +194,9 @@ class Emulator(object):
                 except core.cdist_type.NoSuchTypeError as e:
                     self.log.error("%s requires object %s, but type %s does not exist (definded at %s)"  % (self.cdist_object.name, requirement, e.name, self.object_source))
                     raise
-
+                except core.cdist_object.MissingObjectIdError as e:
+                    self.log.error("%s requires object %s (without object id), but type %s is not a singleton (definded at %s)"  % (self.cdist_object.name, requirement, e.type_name, self.object_source))
+                    raise
 
                 self.log.debug("Recording requirement: " + requirement)