This changes `ListFn`'s implementation so that if it encounters a not
found error, instead of sending a fatal error to log, it coordinates the
return of the error between checker goroutines and sends it back to the
caller.
The main impetus here is that it allows an external program compiling
against rclone as a package to handle a not found, where it currently it
cannot.
This does change error output on a not found a little bit, we go from
this:
2017/01/09 21:14:03 directory not found
To this:
2017/01/09 21:13:44 Failed to ls: directory not found
This works by making sure directory listings that use a filter only
iterate the files provided in the filter (if any).
Single file copies now don't iterate the source or destination
buckets.
Note that this could potentially slow down very long `--files-from`
lists - this is easy to fix (with another flag probably) if it causes
anyone a problem.
Fixes#610Fixes#769
