This commit uses Go's error wrapping features to transparently add some optional
info to certain planfile/state read errors. Specifically, we wrap errors when we
think we've identified the file type but are somehow unable to use it.
Callers that aren't interested in what we think about our input can just ignore
the wrapping; callers that ARE interested can use `errors.As()`.
Having this sitting loose in `cloud` proved problematic because of circular
import dependencies. Putting it in a sub-package under cloud frees us up to
reference the type from places like `internal/backend`!
