Define expect-error as an alias for assert-error.

Define expect-error as an alias for assert-error.

The difference is in intent: expect-error means something is broken,
and the test signals an error because of a bug; assert-error means
the correct behaviour is to signal an error.

Pass expect-error or expect-failure as procedures, not as symbols
representing them, throughout the test suite.

Now we can find documented bugs by grepping for expect-error and
expect-failure.