Move rlcone error.

[JoeZiminski]

EDIT:
on further consideration this is quite complex, as rclone configs are setup up-front. So this would only work in local-only mode, and it is fiddly. As rlcone config is not too onerous it just means the package is not pip-installable, and users that do not transfer are unecessarily downloading a package. It's not ideal but will leave this for now, because changing this is quite deep in the codebase and will require a lot of thinking / checking.

The implementation here is not good, it would be necessary to make a new decorator @requires_rclone.

There is a check that rclone is installed, which raises if it is not installed. Previously this was in the DataShuttle class __init__, however now people will want to use datashuttle for validation / creating folders in local mode and will not need transfer. Therefore, this PR moves the check to the configs-check decorator used before transfer.

The install documentation can be changed, and this will be done on #499 to avoid merge conflicts. No tests are requred.