Add better error detection when calculating slope from profile

src/analysis/forecast_twl.py

@@ -337,10 +337,16 @@ def slope_from_profile(
                 ]
                 if len(end_point_btm) == 0:
                     # If there doesn't seem to be an intersection seaward of the top elevation, return none.
+                    logger.warning("No intersections found seaward of top elevation")
                     return None
                 else:
                     end_points[end_type]["x"] = end_point_btm[0]
 
+    # Ensure that top point is landward of bottom point
+    if end_points["top"]["x"] > end_points["btm"]["x"]:
+        logger.warning("Top point is not landward of bottom point")
+        return None
+
     if method == "end_points":
         x_top = end_points["top"]["x"]
         x_btm = end_points["btm"]["x"]