Up until now assertThrows allows to check the type field of an
exception, which is, however, a custom field introduced in a single
regression test. With the change assertThrows allows to check the
message field of an exception, which is set for standard V8 exceptions
by default.

I use the new assertThrows to refactor test/mjsunit/wasm/divrem-trap.js

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2525313003
Cr-Commit-Position: refs/heads/master@{#41302}