Reconciliation Guess Types error causes web client to hang

[tfmorris]

An error received in response to the "guess types" call at the beginning of reconciliation causes the web client to hang with the spinner up forever.

The response is of the form:

{code: "error", message: "org.apache.http.conn.HttpHostConnectException: Con…] failed: Connection refused (Connection refused)", stack: "org.apache.http.conn.HttpHostConnectException: Con…pClientConnectionOperator.java:142)↵ ... 34 more↵"}

which causes an exception: TypeError: Cannot read property 'length' of undefined
here:

OpenRefine/main/webapp/modules/core/scripts/reconciliation/standard-service-panel.js

Lines 55 to 57 in 6095c44

	self._types = data.types;
	if (self._types.length === 0 && "defaultTypes" in self._service) {

Current Results

The web client gets an error and hangs with the spinner up until the page is refreshed.

Expected Behavior

The user gets a meaningful error message explaining what happened.

Additional Context

The reconciliation_service_skeleton example reconciliation service doesn't work with Python 3 and can be used to reproduce the issue.