catch null result from ground motion forecasting

src/seismic_hazard_forecasting.py

@@ -462,6 +462,11 @@ verbose: {verbose}")
         end = timer()
         logger.info(f"Ground motion exceedance computation time: {round(end - start, 1)} seconds")
 
+        if np.isnan(iml_grid_raw).all():
+            msg = "No valid ground motion intensity measures were forecasted. Try a different ground motion model."
+            logger.error(msg)
+            raise Exception(msg)
+
         # create list of one empty list for each imt
         iml_grid = [[] for _ in range(len(products))]  # final ground motion grids
         iml_grid_prep = iml_grid.copy()  # temp ground motion grids