Raise exception on invalid sliding law in ssa_solver.sliding_law

Author: jrmaddison

fenics_ice/solver.py

@@ -516,6 +516,8 @@ def sliding_law(self, alpha, U):
 
             else:
                 raise RuntimeError(f"Invalid sliding law: '{sl:s}'")
+        else:
+            raise RuntimeError(f"Invalid sliding law: '{sl:s}'")
 
         return B2