Gracefully handle invalid access credentials #26
Description
Recently we implemented a validation step for new S3 storage disks (#4). Now I found an error where apparently the AWS access key became invalid after the storage disk was created (and validated). This caused an nondescript server error again ("The AWS Access Key Id you provided does not exist in our records."). We should implement an error handler for these kinds of errors to show the user a meaningful error message. This has to be done in biigle/core (probably) because the volume file browser is implemented there.