Handle S3 connection failure

[ramanathan106]

.flush() in _EventLoggerThread create a new connection each time, if there is fluctuation in connection boto3.client('s3', endpoint_url) throws an error and since it is not handled the thread will hang and since the queue is full the training will also hang. The try block added will prevent the thread from getting stuck, instead it waits for the connection to appear again. Since it's a while loop the training wont resume till the connection is established again. Connection variable will make sure the print happens only once.

[codecov-io]

Codecov Report

❌ Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.47%. Comparing base (d877dfe) to head (cddad18).
⚠️ Report is 109 commits behind head on master.

Files with missing lines	Patch %	Lines
tensorboardX/event_file_writer.py	41.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
- Coverage   88.68%   88.47%   -0.21%     
==========================================
  Files          38       38              
  Lines        2774     2785      +11     
==========================================
+ Hits         2460     2464       +4     
- Misses        314      321       +7     

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