add API to make it easier to work with ErrorDetail if you have/want a…

…n Any (#409)

If your code already has an instance of `Any`, it's a pain (and not even
guaranteed to be possible at runtime!) to unmarshal the message therein
only to re-wrap it in order to put it in an `ErrorDetail`.

Similarly, if you can't unmarshal the contained type, it may be useful
to access the `Any` envelope.

This adds two new APIs for just this.

error.go

@@ -44,8 +44,14 @@ type ErrorDetail struct {
 	wireJSON string // preserve human-readable JSON
 }
 
-// NewErrorDetail constructs a new error detail.
+// NewErrorDetail constructs a new error detail. If msg is an *[anypb.Any] then
+// it is used as is. Otherwise, it is first marshalled into an *[anypb.Any]
+// value. This returns an error if msg cannot be marshalled.
 func NewErrorDetail(msg proto.Message) (*ErrorDetail, error) {
+	// If it's already an Any, don't wrap it inside another.
+	if pb, ok := msg.(*anypb.Any); ok {
+		return &ErrorDetail{pb: pb}, nil
+	}
 	pb, err := anypb.New(msg)
 	if err != nil {
 		return nil, err