HCL diagnostics relating to expression evaluation include these, so this
is really just to pass that information through our Terraform-specific
diagnostics interface.
Due to the use of interfaces, Diagnostics is not super-friendly to the gob
encoding we currently use for plugin RPC. To mitigate this, we provide
a helper that converts all of the wrapped objects into a predictable flat
structure that we can pre-emptively register with gob.
This means that the decoded Diagnostics still has the same meaning as
the original, though the original wrapped errors (if any) are lost and
thus our errwrap integration won't be effective any longer.
