Student importer script sanity checks

Student importer script sanity checks

1. Skip rows in the CSV file that contain no data for student_id.

2. Avoid trying to create students whose barcode matches an existing
   username in addition to an existing barcode.

3. Avoid inspecting and translating data for non-new accounts when only
   importing new accounts.  This reduces the work load of the script and
   allows it to complete faster.

4. Improve handling/logging for empty files

5. Exit with status 0 for calls to announce with the 'die' flag set to
   true.  In other words, treat them as expected errors that fail
   gracefully without preventing the calling script (sftp-client-agent)
   from exiting on an expected error.

6. Strip non-alpha-numeric characters from student_id values

Signed-off-by: Bill Erickson <berickxx@gmail.com>