Improve config validation for group

[Danielhiversen]

Description:
Before an invalid entity id for groups generated this error message:

16-06-02 15:09:37 ERROR (MainThread) [homeassistant.bootstrap] Invalid config for [group]: Group petersroom is invalid: not a valid value for dictionary value @ data['entities'] for dictionary value @ data['group']

With this change it will be:

16-06-02 15:46:24 ERROR (MainThread) [homeassistant.bootstrap] Invalid config for [group]: Group petersroom is invalid: Entity ID group.tracke--rs is an invalid entity id for dictionary value @ data['entities'] for dictionary value @ data['group']

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[balloob]

Totally missed this, looks good 🐬