Transition expect-functions to "throwing Message"

[haf]

...that way message printers can optionally choose how to format the test data: # runs, # generated datums, # failures, failing input data, succeeding input data, what verbosity things are.

We can initially make this change by letting the different exceptions (skip exception, failure exception) take a Message in their constructors.

This will make the error output possible to colourise, too.

Alternatively we can make tests return values with applicative-based asserts (for parallel/many-results) or monad-based asserts (for first-failing item fails the test).