Refactorings: type of exception thrown and case of exception caught; moved helper function backup_migrate_dropbox_byte_size() inside class; typo in code doc.

Error in changed token handling (introduced in commit 3-5-2020 14:57:36 (d083be35))

Remove unneccesary rename(); place more code in try statement; inline some variables that are only used once.

- Allow for empty sub path

Issue #2060501 by Bryce Fisher-Fleig: Libraries API Module is required but not an explicit dependency

Issue #1326534 by highrockmedia: Fixes 'Backup and Migrate Dropbox leaves copies of files in website root directory'

Issue #1024976 by Richard Eriksson: Sends the tokenized filename to Dropbox instead of the temporary filename