[SPARK-7277] [SQL] Throw exception if the property mapred.reduce.tasks is set to -1

JIRA: https://issues.apache.org/jira/browse/SPARK-7277

As automatically determining the number of reducers is not supported (`mapred.reduce.tasks` is set to `-1`), we should throw exception to users.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #5811 from viirya/no_neg_reduce_tasks and squashes the following commits:

e518f96 [Liang-Chi Hsieh] Consider other wrong setting values.
fd9c817 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into no_neg_reduce_tasks
4ede705 [Liang-Chi Hsieh] Throw exception instead of warning message.
68a1c70 [Liang-Chi Hsieh] Show warning message if mapred.reduce.tasks is set to -1.