[Bug] Network errors are discarded.

If there's an HTTP error, calls to `GraphQL::Client` can return this:

```
KeyError: key not found: "data" (KeyError)
```

E.g., when I try the example in the readme to `https://example.com/graphql`, I expect some kind of network or HTTP error. Instead, I get the above `KeyError`.

I got this when making changes to my infrastructure. It made it very hard to debug problems.

----

I looked at `load_schema`, and found:

```
pry(main)> GraphQL::Client.load_schema(HTTP)
KeyError: key not found: "data" (KeyError)
from gems/3.4.0/gems/graphql-2.5.2/lib/graphql/schema/loader.rb:17:in 'Hash#fetch'
```

The `load_schema` call turns into `load_schema(dump_schema(HTTP))`. `dump_schema` seems to return full error info. So, between those two function calls, the information is lost:

```
[pry(main)> GraphQL::Client.dump_schema(HTTP)
=> {"errors" => [{"message" => "403 Forbidden"}]}
```

This happens other places that call `execute`, I believe, like `.query()`.

----

In my opinion, the core problem is, the library uses fp-style `Either` error returns. However, Ruby doesn't have a way (like Rust or Haskell do) to enforce the handling of the error case. And so, it's possible to simply not handle them appropriately, like here.

Either-style API:

https://github.com/github-community-projects/graphql-client/blob/aff58829ea27c7e59213756aab0bdfdb0e03941a/lib/graphql/client/http.rb#L62-L63

IMO, the fix is to use Ruby's Exception/Error facility so that library clients can meaningfully handle errors.