return better error message when mapper or reducer is null
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Key: MRUNIT-65
URL: https://issues.apache.org/jira/browse/MRUNIT-65
Project: MRUnit
Issue Type: Improvement
Reporter: Jim Donofrio
Priority: Minor
Currently if you use the default constructor for MapDriver or ReduceDriver you
just get a nullpointer exception instead of a more meaningful error stating
that you did not specify a mapper or reducer class
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