Live S3 tests don't handle rate limit errors from AWS

[exarkun]

Build https://travis-ci.org/twisted/txaws/builds/190249354 failed:

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[ERROR]
Traceback (most recent call last):
Failure: twisted.internet.defer.FirstError: FirstError[#0, [Failure instance: Traceback: <class 'txaws.s3.exception.S3Error'>: Error Message: Please reduce your request rate.
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/twisted/web/_newclient.py:1185:_bodyDataFinished_CONNECTED
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/txaws/client/base.py:146:connectionLost
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/twisted/internet/defer.py:457:callback
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/twisted/internet/defer.py:565:_startRunCallbacks
--- <exception caught here> ---
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/twisted/internet/defer.py:651:_runCallbacks
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/txaws/s3/client.py:40:s3_error_wrapper
/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/txaws/client/base.py:65:error_wrapper
]]
txaws.s3.tests.test_client.LiveS3TestCase.test_buckets
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[ERROR]
Traceback (most recent call last):
Failure: twisted.trial.util.DirtyReactorAggregateError: Reactor was unclean.
Selectables:
<<class 'twisted.internet.tcp.Client'> to ('s3.amazonaws.com', 443) at 7f0463707cd0>
txaws.s3.tests.test_client.LiveS3TestCase.test_buckets
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/txaws/s3/client.py", line 40, in s3_error_wrapper
    error_wrapper(error, S3Error)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/txaws/client/base.py", line 65, in error_wrapper
    raise fallback_error
txaws.s3.exception.S3Error: Error Message: Please reduce your request rate.
txaws.s3.tests.test_client.LiveS3TestCase.test_objects
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