Fix mapcache not having permission to write to cache directory

mapserver/cgi-bin/layer_times.py

 #!/usr/bin/env python3
+import os
 import cgi
 import json
 import fiona
 
 form = cgi.FieldStorage()
 layer = form['layer'].value
-with fiona.open(layer, 'r') as shp_file:
-    times = [x['properties']['time'] for x in shp_file]
+if not os.path.isfile(layer):
+    times = []
+else:
+    with fiona.open(layer, 'r') as shp_file:
+        times = [x['properties']['time'] for x in shp_file]
 
 print("Content-Type: application/json")
 print("Access-Control-Allow-Origin: *")