Fix variable names to give context for translators

fix other various i18n issues

lms/static/coffee/src/instructor_dashboard/analytics.coffee

@@ -34,7 +34,9 @@ class ProfileDistributionWidget
     @reset_display()
 
     @get_profile_distributions @feature,
-      error: std_ajax_err => @show_error gettext("Error fetching distribution.")
+      error: std_ajax_err =>
+          `// Translators: "Distribution" refers to a grade distribution. This error message appears when there is an error getting the data on grade distribution.`
+          @show_error gettext("Error fetching distribution.")
       success: (data) =>
         feature_res = data.feature_results
         if feature_res.type is 'EASY_CHOICE'

lms/static/coffee/src/instructor_dashboard/membership.coffee

@@ -142,7 +142,7 @@ class AuthListWidget extends MemberListWidget
       data: rolename: @rolename
       success: (data) => cb? null, data[@rolename]
       error: std_ajax_err => 
-        `// Translators: A rolename appears this sentence.`
+        `// Translators: A rolename appears this sentence. A rolename is something like "staff" or "beta tester".`
         cb? gettext("Error fetching list for role") + " '#{@rolename}'"
 
   # send ajax request to modify access
@@ -453,7 +453,7 @@ class BatchEnrollment
         (sr.identifier for sr in notenrolled)
 
     if notunenrolled.length
-      `// Translators: A list of users appears after this sentence`
+      `// Translators: A list of users appears after this sentence. This situation arises when a staff member tries to unenroll a user who is not currently enrolled in this course.`
       render_list gettext("These users were not affiliated with the course so could not be unenrolled:"),
         (sr.identifier for sr in notunenrolled)