Better error when invalid URL passed into `krane login`

When running the below command the CLI will error with `Error: Unable to authenticate with example.com`

```
$ krane login example.com
```

This happens because the URL does not contain `http` or `https`. We should instead either default to https or throw an error when an invalid URL is passed in.

# Fix

Add `http` or `https` 

```
$ krane login https://example.com
```