Raise OSError exception instead of exiting

OSError is more specific than SystemExit. It is better to have a more
specific exception when we want to catch it.

Common/util_eddies.py

@@ -15,7 +15,8 @@ class SHPC_class:
         """
 
         if not os.access(SHPC_dir, os.R_OK):
-            sys.exit(f"{SHPC_dir} not readable")
+            print(f"{SHPC_dir} not readable")
+            raise OSError
 
         self.dir = SHPC_dir
         self.def_orient = def_orient