Raise an exception in Canvas WPT generator for invalid uses of expected

The 'expected' param is currently only supported for non-grid
testharness tests. This parameter adds an image to testharness test
pages that can be used to visually compare the test results with the
expectation. This image however isn't used by the test runner to
automatically validate the test result. As such, its value is quite
limited. Support for this parameter was therefore not extended to
newer features like reference tests and variant tests.

Bug: 364549423
Change-Id: I30a33dd3634cf0d4cd98cc71b224bcdd1b5de3d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5839991
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361218}