This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
When using `--baseurl` before this patch, if a request was made to the
base URL without a trailing / then rclone would return a 404 error.
Unfortunately GVFS / Nautilus makes the request without the /
regardless of what the user put in.
This patch redirects the request to the base URL with a /. So if the
user was using `--baseurl rclone` then a request to
http://localhost/rclone would be redirected with a 308 response to
http://localhost/rclone/Fixes#4814
Before this change using --user and --pass was impossible on the rc
from a browser as the browser needed to make the OPTIONS request first
before sending Authorization: headers, but the OPTIONS request
required an Authorization: header.
After this change we allow OPTIONS requests to go through without
checking the Authorization: header.
