Improve error messages

[mathstuf]

I'm seeing error messages like operation type not in schema. Getting the operation type name in there would be handy :) .

[tomhoule]

Good point, it shouldn't be too hard to do either. We should probably have a longer, more explanatory message that says:

• operation type not in schema
• what the operation we did not find is
• what operations are there
• what to do about it

I'll probably manage to find time to do this soon, if someone wants to contribute this it's even better :)

[mathstuf]

I'll try and take a look at it this weekend, but I can't guarantee anything.

[fnune]

Regarding this error:

            panic!(
                "operation type '{:?}' not in schema",
                operation.operation_type
            );

I can't manage to trigger that through the GitHub example - operation types are query, mutation, subscription or fragment right? The parser fails before that error is reachable. Maybe I'm misunderstanding something.