Fix undefined names: exc_info --> self.exc_info

[cclauss]

Without this fix exc_info is an undefined name which might result in NameError being raised instead of the desired error.

flake8 testing of https://github.com/apache/beam

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./sdks/python/apache_beam/runners/worker/data_plane.py:185:19: F821 undefined name 'exc_info'
            raise exc_info[0], exc_info[1], exc_info[2]
                  ^
./sdks/python/apache_beam/runners/worker/data_plane.py:185:32: F821 undefined name 'exc_info'
            raise exc_info[0], exc_info[1], exc_info[2]
                               ^
./sdks/python/apache_beam/runners/worker/data_plane.py:185:45: F821 undefined name 'exc_info'
            raise exc_info[0], exc_info[1], exc_info[2]
                                            ^

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[aaltay]

LGTM. Thank you.