Add basic error handling to GraphQL query

[ComputerDruid]

Fixes #123

When I use a fine-grained token now, I get:

thread 'main' panicked at 'API Error 401 Unauthorized: {"documentation_url":"https://docs.github.com/graphql/guides/forming-calls-with-graphql#authenticating-with-graphql","message":"Personal access tokens with fine grained access do not support the GraphQL API"}', src/main.rs:177:13