Improve error messages

[Anviking]

• lazy
• fast
• lots of stuff
• display the important stuff
• automatic

We could retrieve type information in each => overload / equivalent
Large dictionaries/arrays could be stripped down to show only a few bordering items for the description representation.

[Anviking]

More concrete first steps:

• Store references to each dictionary inside the Error, not just the path
• Create custom string representations using this

Personally I think some kind of tweaked yaml style format would be useful showing:

• The keys leading to the error
• Either:
  • The object whose key is missing
  • The key that is missing
  • The object whose key is missing

For example | could indicate an error:

source: 
|        id: 163986909
|        actor: 
|        |     organizations_url: https://api.github.com/users/Anviking/orgs
|        |     repos_url: https://api.github.com/users/Anviking/repos
|        |     html_url: https://github.com/Anviking
|        |     site_admin: 0
|        |     gravatar_id: 
|        |     starred_url: https://api.github.com/users/Anviking/starred{/owner}{/repo}
|        |     avatar_url: https://avatars.githubusercontent.com/u/304423?v=3
|        |     type: User
|        |     gists_url: https://api.github.com/users/Anviking/gists{/gist_id}
|        |    |login: 42 <---- Type mismatch - expected String, not Int.
|        |     followers_url: https://api.github.com/users/Anviking/followers
|        |     id: 304423
|        |     subscriptions_url: https://api.github.com/users/Anviking/subscriptions
|        |     following_url: https://api.github.com/users/Anviking/following{/other_user}
|        |     received_events_url: https://api.github.com/users/Anviking/received_events
|        |     events_url: https://api.github.com/users/Anviking/events{/privacy}
|        |     url: https://api.github.com/users/Anviking
other stuff with no errors
        foo
        bar