[SPARK-6975][Yarn] Fix argument validation error

[jerryshao]

numExecutors checking is failed when dynamic allocation is enabled with default configuration. Details can be seen is SPARK-6975. @sryza, please help me to review this, not sure is this the correct way, I think previous you change this part :)

[srowen]

enabled -> enabled. I think this is simpler to state as "Number of executors was $numExecutors, but must be at least 1 (or 0 if dynamic executor allocation is enabled)."

[jerryshao]

OK, I will change the statement.

[SparkQA]

Test build #30466 has finished for PR 5551 at commit 77bdcbd.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[SparkQA]

Test build #30471 has finished for PR 5551 at commit 4335da1.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[srowen]

Jenkins, retest this please.

[SparkQA]

Test build #30481 has finished for PR 5551 at commit 4335da1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[vanzin]

Looks fine to me. Seems like it might be useful (and easy) to have different messages for each case, but not a big deal.

[andrewor14]

This looks fine. The previous setting was very confusing to the user because they would have run into this by following the docs. Thanks for fixing this I'm merging this into master and 1.3