Add `ErrorIs` matcher to EvalJsResult

This creates a central matcher for use with errors, and removes some
matcher usage that directly references to EvalJsResult::error (which
will make it easier to make that field private, someday). The central
matcher makes it easier to make more complex assertions against errors.

Change-Id: I72931b4442f48273e09ecb10a59d3abca1ccfe7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781572
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491619}