billing: Better error message for dev on lacking Stripe credentials.

Normal server admins will never run this code, and zulipchat.com will
have this information configured before users see it, so this message
is really just for development.

zilencer/views.py

@@ -123,6 +123,10 @@ def add_payment_method(request: HttpRequest) -> HttpResponse:
         ctx["error_message"] = _("You should be an administrator of the organization %s to view this page."
                                  % (user.realm.name,))
         return render(request, 'zilencer/payment.html', context=ctx)
+    if STRIPE_PUBLISHABLE_KEY is None:
+        # Dev-only message; no translation needed.
+        ctx["error_message"] = "Missing Stripe config. In dev, add to zproject/dev-secrets.conf ."
+        return render(request, 'zilencer/payment.html', context=ctx)
 
     try:
         if request.method == "GET":