Change DuplicateFlagCannotPropagateNoneToSwig's super class to FlagsError.
Change DuplicateFlagCannotPropagateNoneToSwig's super class to FlagsError.

Technically it's not a DuplicateFlag error, this also allows us removing the DuplicateFlag class.

	Change on 2016/09/16 by yileiyang <yileiyang@google.com>

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133410796

google/python-gflags