Improve screenshot action error handling

If the gradle build fails for some reason, the script should
bail out instead of carrying on and pushing a commit that
deletes all the screenshots[1]!

`set -e` simply makes the script return the appropriate
exit code immediately if any of the subcommands fails.

[1] e.g.
https://github.com/vector-im/element-x-android/pull/726/commits/a58834240e1054688527955ea3414d8687bd5975

.github/workflows/scripts/recordScreenshots.sh

@@ -16,6 +16,8 @@
 # limitations under the License.
 #
 
+set -e
+
 TOKEN=$GITHUB_TOKEN
 REPO=$GITHUB_REPOSITORY