*Background:* The verification workflow assumes that all verified courses will have a price. Free verified course modes cause a 404 when the user attempts to enroll or upgrade.
This pull request adds a validation check to the `CourseMode` model to check that a price has been set if the mode is set to `validation` or `professional`.
*Testing:* Create a `CourseMode` instance in the django admin with mode `verified` and price 0.
Expected: fails with a validation error
Previous behaviour: accepted. Attempt to upgrade to verified course results in 404:
<img width="870" alt="screen shot 2016-01-29 at 13 58 47" src="https://cloud.githubusercontent.com/assets/793379/12669187/1c3097be-c691-11e5-92be-87d34e3d4a52.png">
Click 'Upgrade to Verified':
<img width="632" alt="screen shot 2016-01-29 at 13 59 35" src="https://cloud.githubusercontent.com/assets/793379/12669188/1c362256-c691-11e5-942d-13ae65067e10.png">