Turn off continue-on-error for browser jobs

I _think_ this is preventing us from receiving email notifications when
one of the browser jobs fails.

The motivation for the current configuration is not clear to me, but I’m
guessing the idea was that the failure of a single browser test run
shouldn’t cancel the others. Turning off fail-fast should do the same
thing.

.github/workflows/test-browser.yml

@@ -8,8 +8,8 @@ on:
 jobs:
   test-browser:
     runs-on: ubuntu-20.04
-    continue-on-error: true
     strategy:
+        fail-fast: false
         matrix:
             browser: [chromium, firefox, webkit]
     steps: