* It turns out that Test::Unit::AssertionFailedError is derived from
StandardError; whereas MiniTest::Assertion is derived from Exception.
* I had been telling Mocha::ExpectationErrorFactory to use
Test::Unit::AssertionFailedError in the Test::Unit adapter, but since
this inherits from StandardError, this would mean a regression on #15.
* So instead I've introduced a custom exception handler in the new
Test::Unit adapter to rescue Mocha::ExpectationError as if it were a
Test::Unit::AssertionFailedError, even though the former is derived
* Unfortunately I have had to call some private methods in Test::Unit
from within the exception handler method. I'm going to ask the Test::Unit
developers whether there is an alternative to doing this.