[SPARK-17650] malformed url's throw exceptions before bricking Executors

## What changes were proposed in this pull request?

When a malformed URL was sent to Executors through `sc.addJar` and `sc.addFile`, the executors become unusable, because they constantly throw `MalformedURLException`s and can never acknowledge that the file or jar is just bad input.

This PR tries to fix that problem by making sure MalformedURLs can never be submitted through `sc.addJar` and `sc.addFile`. Another solution would be to blacklist bad files and jars on Executors. Maybe fail the first time, and then ignore the second time (but print a warning message).

## How was this patch tested?

Unit tests in SparkContextSuite

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15224 from brkyvz/SPARK-17650.