docs: Add a new heading section on Casper debugging.

Also enhance the error output from the casper tool.

docs/testing/testing-with-casper.md

@@ -19,6 +19,22 @@ from `frontend_tests/casper_tests/`.
 
 ## Debugging Casper.JS
 
+When a Casper test fails, the first things to check (before you bother
+trying to use the Casper debugging tools are:
+
+* Does your branch actually work if you just open the webapp and try
+  to follow the flow being tested?  Often the answer is no, and you'll
+  find the debugging experience in your browser to be a much more
+  convenient way to fix the issue.
+* Does your branch use ES6 syntax like arrow functions in a context
+  that isn't transpiled (i.e. non-TypeScript code)?  Casper uses the
+  PhantomJS browser, which doesn't support ES6 syntax, so use of
+  non-transpiled ES6 syntax will generally be first discovered via the
+  Casper tests failing..
+* Are there any errors in `var/casper/server.log` (from the root of
+  your `zulip` checkout?  That's where the `run-dev.py` console output
+  goes when running Casper tests.
+
 Casper.js (via PhantomJS) has support for remote debugging. However, it
 is not perfect. Here are some steps for using it and gotchas you might
 want to know; you'll likely also want to read the section on writing

frontend_tests/run-casper

@@ -106,9 +106,8 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
                 break
     if ret != 0:
         print("""
-Oops, the frontend tests failed. Tips for debugging:
- * Check the screenshots of failed tests at var/casper/casper-failure*.png
- * Try remote debugging the test web browser as described in docs/testing/testing-with-casper.md
+The Casper frontend tests failed!  For help debugging, read:
+  https://zulip.readthedocs.io/en/latest/testing/testing-with-casper.html
 """, file=sys.stderr)
 
         sys.exit(ret)