ftp: clean up error handling

ftp: clean up error handling

We don't want to abort on errors since this is an interactive
tool; while at it, also clean up duplicate error reporting on
errors in 'ls' and weird bytes formatting on errors.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: let unexpected exceptions propgate]
[rlb@defaultvalue.org: use str for exception messages]
[rlb@defaultvalue.org: render paths via path_msg()]
[rlb@defaultvalue.org: add match_rx_grp and test for expected output]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>