Fix read and delete functions

[Morganamilo]

When a resource was deleted, the read function would report no change instead of setting the state to null.

The delete function would throw an error if the resource had already been deleted, instead of just continuing.

[graham-russell]

Just to understand, when you say "When a resource was deleted,", does this refer to a resource being removed by a process other than terraform?

[Morganamilo]

Yep.