[SPARK-8893] Add runtime checks against non-positive number of partitions

https://issues.apache.org/jira/browse/SPARK-8893

> What does `sc.parallelize(1 to 3).repartition(p).collect` return? I would expect `Array(1, 2, 3)` regardless of `p`. But if `p` < 1, it returns `Array()`. I think instead it should throw an `IllegalArgumentException`.

> I think the case is pretty clear for `p` < 0. But the behavior for `p` = 0 is also error prone. In fact that's how I found this strange behavior. I used `rdd.repartition(a/b)` with positive `a` and `b`, but `a/b` was rounded down to zero and the results surprised me. I'd prefer an exception instead of unexpected (corrupt) results.

Author: Daniel Darabos <darabos.daniel@gmail.com>

Closes #7285 from darabos/patch-1 and squashes the following commits:

decba82 [Daniel Darabos] Allow repartitioning empty RDDs to zero partitions.
97de852 [Daniel Darabos] Allow zero partition count in HashPartitioner
f6ba5fb [Daniel Darabos] Use require() for simpler syntax.
d5e3df8 [Daniel Darabos] Require positive number of partitions in HashPartitioner
897c628 [Daniel Darabos] Require positive maxPartitions in CoalescedRDD

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