Thread types through in error messages

[parsonsmatt]

This PR keeps track of the types that are being acquired with getters. I'm going to post a new error message once I've done it on our repo.

Resolves #63

[parsonsmatt]

Eh, this doesn't seem to do much to help for the specific case I was working on.

[stepcut]

@parsonsmatt Even though it did not help your specific case, do you still believe it is worth pulling?

[parsonsmatt]

I'm not sure when this will actually fire, to be honest. I expected that the decoding would keep the context of what type we're in when we go to do another type. eg:

data T = T { x :: X }
data X = X  { a :: A }
data A = A { foo :: Text }

Would display something like:

failed to decode `Text`. Context: A, X, T

But it doesn't do this. So I'm unsure what this actually does.