Remove SparkPandasMergeError

[floscha]

As requested in #264, this PR removes the specific SparkPandasMergeError and replaces its usage with the more generic ValueError.

[codecov-io]

Codecov Report

Merging #323 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
- Coverage   93.88%   93.87%   -0.01%     
==========================================
  Files          34       34              
  Lines        3350     3347       -3     
==========================================
- Hits         3145     3142       -3     
  Misses        205      205

Impacted Files	Coverage Δ
databricks/koalas/exceptions.py	78.12% <ø> (-1.29%)	⬇️
databricks/koalas/tests/test_dataframe.py	100% <100%> (ø)	⬆️
databricks/koalas/frame.py	94.89% <100%> (-0.02%)	⬇️

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[rxin]

Thanks for the quick response. Merging.