Add support for multiple differs

Updates the differ service to support calling and configuring
multiple differs. The differs are configured as an ordered list
of differs which will each be attempting until a supported differ
is called.

Additionally a not supported error type was added to allow differs
to be selective of whether the differ arguments are supported by
the differ. This error type corresponds to the GRPC unimplemented error.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

errdefs/errors.go

@@ -26,6 +26,7 @@ var (
 	ErrAlreadyExists      = errors.New("already exists")
 	ErrFailedPrecondition = errors.New("failed precondition")
 	ErrUnavailable        = errors.New("unavailable")
+	ErrNotSupported       = errors.New("not supported") // represents not supported and unimplemented
 )
 
 func IsInvalidArgument(err error) bool {
@@ -52,3 +53,7 @@ func IsFailedPrecondition(err error) bool {
 func IsUnavailable(err error) bool {
 	return errors.Cause(err) == ErrUnavailable
 }
+
+func IsNotSupported(err error) bool {
+	return errors.Cause(err) == ErrNotSupported
+}

errdefs/grpc.go

@@ -38,6 +38,8 @@ func ToGRPC(err error) error {
 		return grpc.Errorf(codes.FailedPrecondition, err.Error())
 	case IsUnavailable(err):
 		return grpc.Errorf(codes.Unavailable, err.Error())
+	case IsNotSupported(err):
+		return grpc.Errorf(codes.Unimplemented, err.Error())
 	}
 
 	return err
@@ -69,6 +71,8 @@ func FromGRPC(err error) error {
 		cls = ErrUnavailable
 	case codes.FailedPrecondition:
 		cls = ErrFailedPrecondition
+	case codes.Unimplemented:
+		cls = ErrNotSupported
 	default:
 		cls = ErrUnknown
 	}