CLC-4321 - Investigate possible issues with pending eGrades Export tool

https://jira.ets.berkeley.edu/jira/browse/CLC-4321

Provides validation and appropriate error display when state of course site not appropriate for eGrade Export. Ensures that export uses letter grade provided by Canvas via Grading standard/schemes, and only includes student grades from selected section.
I'm sorry @christianv, I'm afraid I can't do that.
![image](https://cloud.githubusercontent.com/assets/746759/4672788/1f4d28a8-5596-11e4-9a42-c76f5e7bc61c.png)