Use GustoIOError throughout io.py

gusto/io.py

@@ -291,7 +291,7 @@ def setup_dump(self, state_fields, t, pick_up=False):
                 state from a checkpointing file. Defaults to False.
 
         Raises:
-            IOError: if the results directory already exists, and the model is
+            GustoIOError: if the results directory already exists, and the model is
                 not picking up or running in test mode.
         """
 
@@ -310,7 +310,7 @@ def setup_dump(self, state_fields, t, pick_up=False):
                 elif not (running_tests or pick_up):
                     # Throw an error if directory already exists, unless we
                     # are picking up or running tests
-                    raise IOError(f'results directory {self.dumpdir} already exists')
+                    raise GustoIOError(f'results directory {self.dumpdir} already exists')
 
         if self.output.dump_vtus or self.output.dump_nc:
             # make list of fields to dump