Stop `open_shpc` if the directory does not exist

The function would crash anyway because "extremum" is not in
`handler["readers"]` but the error messages would be less clear.

Common/util_eddies.py

@@ -2,6 +2,7 @@ import shapefile
 import math
 from os import path
 import numpy as np
+import sys
 
 def comp_ishape(handler, date, eddy_index):
     k = date - handler["d_init"]
@@ -21,6 +22,9 @@ def in_window(point, window):
     return longit <= urcrnrlon and llcrnrlat <= point[1] <= urcrnrlat
 
 def open_shpc(shpc_dir):
+    if not path.isdir(shpc_dir):
+        sys.exit(f"open_shpc: {shpc_dir} is not an existing directory")
+
     handler = {"readers": {}}
 
     for layer in ["extremum", "outermost_contour", "max_speed_contour",