Handle StopIteration in CloudWatch Logs retrieval during training

[yangaws]

Issue #, if available:

Description of changes:
In current codes, StopIteration exception was not handled when retrieving CloudWatch Log streams during training. That's why there's related warnings 'DeprecationWarning: generator 'multi_stream_iter' raised StopIteration' can be seen in logs. And in py3.7, this will be an error instead of a warning, see:

PEP 479 is enabled for all code in Python 3.7, meaning that StopIteration exceptions raised directly or indirectly in coroutines and generators are transformed into RuntimeError exceptions. (Contributed by Yury Selivanov in bpo-32670.)

This PR handles the exception by swallowing it.

Merge Checklist

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[codecov-io]

Codecov Report

Merging #594 into master will decrease coverage by 0.06%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   92.75%   92.69%   -0.07%     
==========================================
  Files          71       71              
  Lines        5397     5405       +8     
==========================================
+ Hits         5006     5010       +4     
- Misses        391      395       +4

Impacted Files	Coverage Δ
src/sagemaker/logs.py	89.06% <55.55%> (-5.59%)	⬇️

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