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.
@redconfetti can you rewrite this and put everything in 1 if statement?

```
if ($scope.appState !== 'error')
  loadSectionTerms(data.section_terms);
  loadGradeTypes(data.grade_types_present);
...