Better Handling of Boto3 Exceptions

[sean-smith]

Planning on fixing this asap, similar to how it's done in https://github.com/aws/aws-parallelcluster

Here's what a credential error looks like:

Traceback (most recent call last):
  File "/Users/seaam/.pyenv/versions/3.7.0/bin/awsbw", line 10, in <module>
    sys.exit(main())
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/awsbw/awsbw.py", line 724, in main
    wrapper(start, args)
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/awsbw/awsbw.py", line 710, in start
    args.queue,
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/awsbw/awsbw.py", line 46, in __init__
    self.getJobs()
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/awsbw/awsbw.py", line 326, in getJobs
    status=status
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/awsbw/awsbw.py", line 245, in queueJobs
    jobStatus=status,
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/botocore/client.py", line 357, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/Users/seaam/.pyenv/versions/3.7.0/lib/python3.7/site-packages/botocore/client.py", line 661, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (ExpiredTokenException) when calling the ListJobs operation: The security token included in the request is expired

By handling ClientError, this could become:

$ awsbw -Q my-queue
An error occurred (ExpiredTokenException) when calling the ListJobs operation: The security token included in the request is expired