Decide how to report 'unexpected' and 'actual' values in an exception

[marcphilipp]

This is a follow-up on #138 where we used new AssertionFailedError(message, expected, actual) to pass expected and actual for assertEquals, assertSame, and assertNull.

However, we should consider passing unexpected and actual for

• Assertions.assertNotEquals
• Assertions.assertNotSame (hmm... unexpected == actual in this case)
• Assertions.assertNotNull (just actual?)

[jbduncan]

FYI, Google Truth produces IMO rather nice error messages that show how actual and expected differ to each other if they are both strings. Perhaps their error message format could be a source of inspiration for this issue.