Improve error message when @InjectMocks is uses on an interface or enum field

[ChristianSchwarz]

By accident a tests delares a field of an interface type instead of the implementing class.

@InjectMocks
public InterfaceType unitUnderTest;

Injection is not possible on interfaces or enums. The error message should make this clear, the currenta message that doesn't help much:

org.mockito.exceptions.base.MockitoException: 
Cannot instantiate @InjectMocks field named 'configurationManager'.
You haven't provided the instance at field declaration so I tried to construct the instance.
However, I failed because: the type 'ConfigurationManager' is an interface.
Examples of correct usage of @InjectMocks:
   @InjectMocks Service service = new Service();
   @InjectMocks Service service;
   //also, don't forget about MockitoAnnotations.initMocks();
   //and... don't forget about some @Mocks for injection :)

The error message should be someting like:
The field 'unitUnderTest' can not be annotated with @InjectMocks cause the type 'InterfaceType ' not a class!