[SPARK-13211] [STREAMING] StreamingContext throws NoSuchElementException when created from non-existent checkpoint directory

[srowen]

What changes were proposed in this pull request?

Take 2: avoid None.get NoSuchElementException in favor of more descriptive IllegalArgumentException if a non-existent checkpoint dir is used without a SparkContext

How was this patch tested?

Jenkins test plus new test for this particular case

[srowen]

Jenkins retest this please

[SparkQA]

Test build #54980 has finished for PR 12174 at commit 320c932.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

retest this please

[zsxwing]

LGTM pending tests

[SparkQA]

Test build #55023 has finished for PR 12174 at commit 320c932.

• This patch passes all tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[zsxwing]

retest this please

[SparkQA]

Test build #55045 has finished for PR 12174 at commit 320c932.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

Thanks, merging to master