[Import Maps] Fix error reporting in test helper

Just to plumb message correctly.

Bug: 848607
Change-Id: I753963ba5a25da085e60c7ac0bd2f8466aaa3aa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570020Reviewed-by: Domenic Denicola <domenic@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833315}

third_party/blink/web_tests/external/wpt/import-maps/data-driven/resources/test-helper.js

@@ -83,7 +83,7 @@ function resolve(specifier, parsedImportMap, baseURL, expectedURL) {
           if (event.data.result === 'TypeError') {
             reject(new TypeError(event.data.message));
           } else {
-            reject(new Error(event.data.result));
+            reject(new Error(event.data.message));
           }
         } else {
           assert_unreached('Invalid message: ' + event.data.type);