handle already existing remote files on move

Merged anarcat requested to merge error-handling into main

For some reason, we have files in the processing folder that are already present before moving. Skip those files instead of crashing with:

webdav3.exceptions.ResponseErrorCode: Request to https://nc.torproject.net/remote.php/dav/files/dangerzone-bot/CVS/dangerzone-processing/2/ failed with code 412 and message: b'<?xml version="1.0" encoding="utf-8"?>\n<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">\n  <s:exception>Sabre\\DAV\\Exception\\PreconditionFailed</s:exception>\n  <s:message>The destination node already exists, and the overwrite header is set to false</s:message>\n  <s:header>Overwrite</s:header>\n</d:error>\n'

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