Better error message for src;dest

[TwitchBronBron]

Could we have better error messages for certain types of invalid src;dest objects? For example:

src is file and dest is dir

`` { src: "../manifest", dest: "./"}``` This one should be easy to detect. We could run src` through the `glob.hasMagic` method, and if it's not a magic string, check for `fsExtra.isFile` or `fsExtra.isDir`. Then, we know for sure that the path on the right needs to be a path to a file, or if referencing a dir, throw an error saying directories can't be referenced directly.