Fix error code propagation in 'mb gen'.

tools/mb/mb.py

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """MB - the Meta-Build wrapper around GYP and GN
 
 MB is a wrapper script for GYP and GN that can be used to generate build files
 for sets of canned configurations and analyze them.
 """
@@ skipping 104 matching lines @@
     if vals['type'] == 'gn':
       return self.RunGNAnalyze(vals)
     elif vals['type'] == 'gyp':
       return self.RunGYPAnalyze(vals)
     else:
       raise MBErr('Unknown meta-build type "%s"' % vals['type'])
 
   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()
     if vals['type'] == 'gn':
       cmd = self.GNCmd('gen', '<path>', vals['gn_args'])
     elif vals['type'] == 'gyp':
       cmd = self.GYPCmd('<path>', vals['gyp_defines'], vals['gyp_config'])
     else:
       raise MBErr('Unknown meta-build type "%s"' % vals['type'])
 
@@ skipping 407 matching lines @@
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except MBErr as e:
     print(e)
     sys.exit(1)
   except KeyboardInterrupt:
     print("interrupted, exiting", stream=sys.stderr)
     sys.exit(130)