Friendly error if invalid student is entered for due date extension

This minor commit fixes another problem with individual due date extensions mentioned to me by @sarina in #4868: Whenever an invalid student identifier was entered into any of the "student email/username" fields on the instructor "extensions" page, no error would be shown. 

With this change, the instructor will get the friendly error message "Could not find student matching identifier"

This was done for OpenCraft ( cc @antoviaque ) and falls under their organization contributor agreement.
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