Fix error code propagation in 'mb gen'.

tools/mb/mb.py

Index: tools/mb/mb.py
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 0b5775e9f2069fb887144b1deda0ef700a8763d3..bc43654ee03773a328a95e52dd0218cc3bc0b004 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -122,12 +122,11 @@ class MetaBuildWrapper(object):
   def CmdGen(self):
     vals = self.GetConfig()
     if vals['type'] == 'gn':
-      self.RunGNGen(self.args.path[0], vals)
-    elif vals['type'] == 'gyp':
-      self.RunGYPGen(self.args.path[0], vals)
-    else:
-      raise MBErr('Unknown meta-build type "%s"' % vals['type'])
-    return 0
+      return self.RunGNGen(self.args.path[0], vals)
+    if vals['type'] == 'gyp':
+      return self.RunGYPGen(self.args.path[0], vals)
+
+    raise MBErr('Unknown meta-build type "%s"' % vals['type'])
 
   def CmdLookup(self):
     vals = self.GetConfig()