SPARK-1469: Scheduler mode should accept lower-case definitions and have...

... nicer error messages

There are  two improvements to Scheduler Mode:
1. Made the built in ones case insensitive (fair/FAIR, fifo/FIFO).
2. If an invalid mode is given we should print a better error message.

Author: Sandeep <sandeep@techaddict.me>

Closes #388 from techaddict/1469 and squashes the following commits:

a31bbd5 [Sandeep] SPARK-1469: Scheduler mode should accept lower-case definitions and have nicer error messages There are  two improvements to Scheduler Mode: 1. Made the built in ones case insensitive (fair/FAIR, fifo/FIFO). 2. If an invalid mode is given we should print a better error message.