Raise exception instead of setting invalid netbox api version

This method would previously set netbox_api_version to 0, which is not a valid version that can be parsed. 

This does not fix the issue, but raises this exception with a better message to help the user diagnose

See #1032

plugins/inventory/nb_inventory.py

@@ -1494,7 +1494,7 @@ def fetch_api_docs(self):
             status = self._fetch_information(self.api_endpoint + "/api/status")
             netbox_api_version = ".".join(status["netbox-version"].split(".")[:2])
         except Exception:
-            netbox_api_version = 0
+            raise Exception("Failed to retrieve /api/status or parse netbox_api_version")
 
         tmp_dir = os.path.split(DEFAULT_LOCAL_TMP)[0]
         tmp_file = os.path.join(tmp_dir, "netbox_api_dump.json")