raise error if alias S is not defined

pygmt/base_plotting.py

@@ -2136,12 +2136,10 @@ def velo(self, data=None, **kwargs):
             extra_arrays.append(kwargs["G"])
             del kwargs["G"]
 
-        if scaling is not None:
-            if not isinstance(scaling,str):
+        if "S" in kwargs and not isinstance(kwargs["S"],str):
                 raise GMTInvalidInput(
-                    "scaling has to be a string."
+                    "Scaling is a required and has to be a string."
                 )
-            extra_arrays.append(scaling)
 
         with Session() as lib:
             # Choose how data will be passed in to the module