use LeaderNotFoundError instead of LeaderNotAvailable

[emmettbutler]

After a full audit of the use of ProtocolClientError subclasses in the pykafka codebase, this pull request changes the only instance of inappropriate broker exception use outside of the one mentioned in #697.

[codecov-io]

Codecov Report

Merging #714 into master will increase coverage by 0.25%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage    84.1%   84.36%   +0.25%     
==========================================
  Files          26       26              
  Lines        3373     3416      +43     
  Branches      539      540       +1     
==========================================
+ Hits         2837     2882      +45     
+ Misses        393      390       -3     
- Partials      143      144       +1

Impacted Files	Coverage Δ
pykafka/cluster.py	71.3% <0%> (ø)	⬆️
pykafka/topic.py	79.78% <100%> (ø)	⬆️
pykafka/exceptions.py	100% <100%> (ø)	⬆️
pykafka/partition.py	74.6% <50%> (ø)	⬆️
pykafka/connection.py	77.65% <0%> (-2.13%)	⬇️
pykafka/simpleconsumer.py	82.84% <0%> (-0.5%)	⬇️
pykafka/balancedconsumer.py	90.26% <0%> (+0.29%)	⬆️
pykafka/rdkafka/producer.py	88.49% <0%> (+0.63%)	⬆️
... and 2 more

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