Make GN args error more specific

Bug:

Change-Id: I86774c4f6f9712748cc637f4862cce91ee544159
Reviewed-on: https://chromium-review.googlesource.com/462027Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Jeff Carpenter <jeffcarp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#460457}

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/factory.py

@@ -143,8 +143,10 @@ def _check_configuration_and_target(host, options):
 
     gn_configuration = _read_configuration_from_gn(host, options)
     if gn_configuration:
-        if getattr(options, 'configuration') not in (None, gn_configuration):
-            raise ValueError('Configuration does not match the GN build args.')
+        expected_configuration = getattr(options, 'configuration')
+        if expected_configuration not in (None, gn_configuration):
+            raise ValueError('Configuration does not match the GN build args. '
+                             'Expected "%s" but got "%s".' % (gn_configuration, expected_configuration))
         options.configuration = gn_configuration
         return