win ninja: build fix for msvs_pre/postbuild error test

pylib/gyp/msvs_emulation.py

Index: pylib/gyp/msvs_emulation.py
diff --git a/pylib/gyp/msvs_emulation.py b/pylib/gyp/msvs_emulation.py
index 618478d1c9fe7398a465fc892ff3d168b81aefb7..709ba305d2828e5546007c10c0dc1bc441b98662 100644
--- a/pylib/gyp/msvs_emulation.py
+++ b/pylib/gyp/msvs_emulation.py
@@ -167,12 +167,14 @@ class MsvsSettings(object):
         'msvs_prebuild',
         'msvs_postbuild',
     ]
+    unsupported = []
     for field in unsupported_fields:
       for config in configs.values():
         if field in config:
-          print "Warning: %s not supported, dropping. (target %s)" \
-                % (field, spec['target_name'])
-          break
+          unsupported += ["%s not supported (target %s)." %
+                          (field, spec['target_name'])]
+    if unsupported:
+      raise Exception('\n'.join(unsupported))
 
   def GetVSMacroEnv(self, base_to_build=None, config=None):
     """Get a dict of variables mapping internal VS macro names to their gyp