rados: add error for function args that must have data

Signed-off-by: John Mulligan <jmulligan@redhat.com>

rados/errors.go

@@ -50,6 +50,9 @@ func getErrorIfNegative(ret C.int) error {
 var (
 	// ErrNotConnected is returned when functions are called without a RADOS connection
 	ErrNotConnected = errors.New("RADOS not connected")
+	// ErrEmptyArgument may be returned if a function argument is passed
+	// a zero-length slice or map.
+	ErrEmptyArgument = errors.New("Argument must contain at least one item")
 )
 
 // Public radosErrors: