Bug 1370535 - Provide a useful error message when passing --enable-geckodriver in artifact bulds. r=froydnj

MozReview-Commit-ID: 88TlmFAdOuf

--HG--
extra : rebase_source : 663ebb22aeafb85a0b2adb163b12b4d08b6d19a0

toolkit/moz.configure

@@ -1130,6 +1130,10 @@ def geckodriver(enable, automation, compile_env, cross_compile, hazard, target):
     default on supported CI build platforms.
     """
     if enable:
+        if not compile_env:
+            die("--enable-geckodriver is not available without a compile "
+                "environment. A geckodriver binary will be downloaded during "
+                "an artifact build by default where available.")
         return True
 
     if enable.origin == 'default':