Make FileSystemProviderApiTest.Evil easier to debug.

Instead of timing out when reading of the file fails, actually report
an error.

Bug: none
Change-Id: I1a32f3b1bc1361eb61ba17d86e67bf8d05692033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974394Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726179}

chrome/test/data/extensions/api_test/file_system_provider/evil/test.js

@@ -262,6 +262,9 @@ function runTests() {
                 var text = fileReader.result;
                 chrome.test.assertEq(0, text.length);
               });
+              fileReader.onerror = function(error) {
+                chrome.test.fail(error.name);
+              };
               fileReader.readAsText(fileSlice);
             }),
             function(error) {