feat: error message for invalid variants

[EverlastingBugstopper]

fixes #208 by providing a helpful error message when trying to fetch an invalid variant

when we find a variant that's pretty close to a variant that exists in the registry:

$ rover graph fetch averys-federated-graph@whatatypoo
Fetching SDL from averys-federated-graph@whatatypoo using credentials from the default profile.
error: The graph registry does not contain variant "whatatypoo" for graph "averys-federated-graph"
        Did you mean "averys-federated-graph@whatatypo"?

when there's only one variant in the registry and the provided variant isn't similar to any of them

$ rover graph fetch averys-federated-graph@notevenclose
Fetching SDL from averys-federated-graph@dne using credentials from the default profile.
error: The graph registry does not contain variant "dne" for graph "averys-federated-graph"
        The only existing variant for graph "averys-federated-graph" is "current".

when there are only two variants in the registry and the provided variant isn't similar to any of them

$ rover graph fetch averys-federated-graph@notevenclose
Fetching SDL from averys-federated-graph@dne using credentials from the default profile.
error: The graph registry does not contain variant "dne" for graph "averys-federated-graph"
        The only existing variants for graph "averys-federated-graph" are "current" and "dev".

when there are between 3 and 10 variants in the registry and the provided variant isn't similar to any of them

$ rover graph fetch averys-federated-graph@notevenclose
Fetching SDL from averys-federated-graph@dne using credentials from the default profile.
error: The graph registry does not contain variant "dne" for graph "averys-federated-graph"
        The only existing variants for graph "averys-federated-graph" are "current", "dev", and "staging".

when there are over 10 variants in the registry

$ rover graph fetch averys-federated-graph@notevenclose
Fetching SDL from averys-federated-graph@cdahksjaksjdh using credentials from the default profile.
error: The graph registry does not contain variant "cdahksjaksjdh" for graph "averys-federated-graph"
        You can view the variants for graph "averys-federated-graph" by visiting https://studio.apollographql.com/graph/averys-federated-graph/settings

[JakeDawkins]

I realize this definitely would've been better feedback on the issue rather than its PR, but I don't know if I like a top-level variant command. I feel like variant is an overloaded term in studio kinda, and without a clear understanding of all the possible operations on a variant, I'm not sure I really feel good about adding a variant command.

What if we added this functionality under a graph info command along with some more info that'd be useful about a graph?

I'm happy to sketch out more of this idea in that issue too :)

[EverlastingBugstopper]

blech i force pushed something into a merge commit. gotta double make sure we squash and merge this one lol.

[JakeDawkins]

Love the use of frontend_url_root 🙌