Add optional error/result outparam to highlevel filedat API functions

- loadfile
- addfile
- removefile
- updatefile

As discovered via ftp server "error updating" file log message, no details
could be logged since these functions just returned a simple bool. So, we
now support passing an optional output parameter to capture the result/error
value to be displayed or logged when these functions return false.

Unrelated fixes to start_batch_download():
When passing a list of files on the command to the file transfer protocol
driver (e.g. sz), if any filename contains spaces, that filename must be
quoted or else sz will fail with error 'sz: can read only one file from stdin'.
Also, the trailing whitespace from the list of filenames must be removed or
else sz will report errors 'sz: cannot open : No such file or directory'
and 'sz: Transfer incomplete'.