Show original statement if assert or refute fails

If the asserted statement evaluates to false, show the unevaluated
statement in the assertion error output message for more context about
the failure. Refute also now follows similar semantics.

Fixes #1276
Merge pull request #1299 from vanstee/assert-statement-if-boolean

Show original statement if assert or refute fails
Fix misspelling of "Instead"
Include description and improve formatting

Switching from `AssertionError` to `ExpectationError` gives us a nicer
error message and allows us to easily include the incorrect result as a
description.

Fixes #1276