URLs without a scheme don't work

[shanepelletier]

I'm undecided how this should be fixed, since surl 0.8 also doesn't work without a scheme, but it does at least return a more useful error message stating such. SCA doesn't work without the scheme specified, so "dashboard.staging.snapcraft.io" errors (specifically in surl v1, it complains about the authorization header) but "https://dashboard.staging.snapcraft.io" works fine.

Three possible solutions are:

1. Detect when the scheme is missing and error
2. Detect when the scheme is missing, add "http://" (which gets automatically redirected to "https://" for the deployed services and would also work for local services), and warn the user that the URL they used was modified to work
3. Detect the scheme is missing and add "http://"

There's a possibility that solution 3 is the easiest, if curl has a command-line option to automatically do the fixing for us.