[api] Improve CheckCast error messages

The error messages previously gave the impression that a cast in the
sense of a JS type conversion may have been attempted.

Clarify the error messages to a) indicate that the type mismatched
(rather than using the term “conversion”) and b) always use the
C++ class name to clarify that the check refers to the actual
class hierarchy, rather than conceptual types.

Change-Id: I28c1b8a265f3d96f344bc2e00ac24bd9b9b6d269
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1987247Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65601}