Display clearer error messages when Wikibase instance is unreachable

Currently running OpenRefine directly from the master branch and reconciling with an independent Wikibase instance doesn't fully work on Chrome on MacOS Catalina.

To Reproduce

Steps to reproduce the behavior:

1. First, run OR in terminal from cloned master branch
2. Then, add reconcilitation service and corresponding manifest
3. Finally, try adding some data and building a schema to prepare to upload the data to Wikibase

Current Results

Once you get to the statement level for a new item – it is not possible to add values to the statement, the value field does not appear.

Expected Behavior

A value box should appear, once a property is selected. At the moment I can select a property, but then nothing happens.

Screenshots

Versions

• Operating System: MacOS Catalina 10.15.6
• Browser Version: Google Chrome Version 84.0.4147.135 (Official Build) (64-bit)
• JRE or JDK Version: JRE 1.8.0_261
• OpenRefine: OpenRefine master branch as of 28.08.2020

No files, tests, or entry points are named. Reproduce the Wikibase reconciliation flow from the cloned master branch in Chrome on macOS, then trace what happens after selecting a property when the instance is unreachable. Done means the failure produces a clear error message and the expected value box behavior is addressed.