Don't send DISCARD after ExecAbortError in pipeline

[nickgaya]

Description of change

Don't send DISCARD after ExecAbortError when executing a MULTI/EXEC transaction in the Pipeline class.

The EXECABORT error type was added in Redis 2.6.5 and is returned from an EXEC command to indicate that the transaction was aborted due to an invalid command. It is not necessary to call DISCARD after this error, and doing so causes a "DISCARD without MULTI" error.

Fixes #1300

Pull Request check-list

• Does $ tox pass with this change (including linting)?
• Does travis tests pass with this change (enable it first in your forked repo and wait for the travis build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[codecov-io]

Codecov Report

Merging #1301 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
+ Coverage   92.76%   92.78%   +0.02%     
==========================================
  Files          19       19              
  Lines        6478     6485       +7     
==========================================
+ Hits         6009     6017       +8     
+ Misses        469      468       -1

Impacted Files	Coverage Δ
redis/client.py	86.25% <ø> (+0.04%)	⬆️
tests/test_pipeline.py	100% <100%> (ø)	⬆️

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[andymccurdy]

Merged in 07fec7e. Thanks for finding this!

I had to do some digging to validate this was a good change. I was actually involved in the original discussion with antirez back in 2012 when this code was added. The existing code worked against a branch he was developing to fix an underlying problem. I shipped the redis-py code, then a few days later in the same thread he changed the behavior of the server to automatically discard before replying with EXECABORT. I apparently missed that change in direction.