DoFnRunner _reraise_augmented fails with non-ascii exception messages

`_reraise_augmented` fails to decode non-ascii bytes when re-raising exceptions and hides the original exception.

Full stack trace:
(6d7ae3386f271a8f): Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/batchworker.py", line 582, in do_work
work_executor.execute()
File "/usr/local/lib/python2.7/dist-packages/dataflow_worker/executor.py", line 166, in execute
op.start()
File "dataflow_worker/native_operations.py", line 38, in dataflow_worker.native_operations.NativeReadOperation.start (dataflow_worker/native_operations.c:3175)
def start(self):
File "dataflow_worker/native_operations.py", line 39, in dataflow_worker.native_operations.NativeReadOperation.start (dataflow_worker/native_operations.c:3079)
with self.scoped_start_state:
File "dataflow_worker/native_operations.py", line 44, in dataflow_worker.native_operations.NativeReadOperation.start (dataflow_worker/native_operations.c:2994)
with self.spec.source.reader() as reader:
File "dataflow_worker/native_operations.py", line 54, in dataflow_worker.native_operations.NativeReadOperation.start (dataflow_worker/native_operations.c:2938)
self.output(windowed_value)
File "apache_beam/runners/worker/operations.py", line 154, in apache_beam.runners.worker.operations.Operation.output (apache_beam/runners/worker/operations.c:5783)
cython.cast(Receiver, self.receivers[output_index]).receive(windowed_value)
File "apache_beam/runners/worker/operations.py", line 86, in apache_beam.runners.worker.operations.ConsumerSet.receive (apache_beam/runners/worker/operations.c:3622)
cython.cast(Operation, consumer).process(windowed_value)
File "apache_beam/runners/worker/operations.py", line 339, in apache_beam.runners.worker.operations.DoOperation.process (apache_beam/runners/worker/operations.c:11089)
with self.scoped_process_state:
File "apache_beam/runners/worker/operations.py", line 340, in apache_beam.runners.worker.operations.DoOperation.process (apache_beam/runners/worker/operations.c:11043)
self.dofn_receiver.receive(o)
File "apache_beam/runners/common.py", line 382, in apache_beam.runners.common.DoFnRunner.receive (apache_beam/runners/common.c:10156)
self.process(windowed_value)
File "apache_beam/runners/common.py", line 390, in apache_beam.runners.common.DoFnRunner.process (apache_beam/runners/common.c:10458)
self._reraise_augmented(exn)
File "apache_beam/runners/common.py", line 429, in apache_beam.runners.common.DoFnRunner._reraise_augmented (apache_beam/runners/common.c:11606)
**** step_annotation)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 1521: ordinal not in range(128)

Imported from Jira [BEAM-3014](https://issues.apache.org/jira/browse/BEAM-3014). Original Jira may contain additional context.
Reported by: altay.

Start in apache_beam/runners/common.py at DoFnRunner._reraise_augmented, using the reported stack trace to follow how exceptions are re-raised. Reproduce the failure with a non-ASCII exception message and verify that re-raising no longer causes UnicodeDecodeError or hides the original exception.

Written by the indexing model from the issue text.