Fix compatibility with python3 of clang-include-fixer.py

clang-include-fixer was recently updated to be python3-compatible.
However, an exception handling clause was improperly using the deprecated `message` property of Exception classes, so the code was not yet entirely python3-compatible.

Differential Revision: https://reviews.llvm.org/D70902

clang-tools-extra/clang-include-fixer/tool/clang-include-fixer.py

@@ -211,7 +211,7 @@ def main():
     InsertHeaderToVimBuffer(include_fixer_context, text)
     print("Added #include {0} for {1}.".format(selected, symbol))
   except Exception as error:
-    print(error.message, file=sys.stderr)
+    print(error, file=sys.stderr)
   return