Lack of descriptive message when adding an incorrect schema manifest

[j-sal]

To Reproduce

Steps to reproduce the behavior:

1. First, add access the menu in the top right corner 'Extensions: Wikidata' -> 'Select Wikibase instance' -> 'Add Wikibase'
2. Then, paste/write your manifest containing syntax mistakes (see screenshot)
3. Finally, select 'Add Wikibase' in the bottom right corner of the window

Current Results

The error "Invalid Wikibase manifest." appears in red letters.

Expected Behavior

A more descriptive error would help narrow down and fix the mistakes.

Screenshots

Versions

• Operating System: Ubuntu 18.04
• Browser Version: Chrome 95
• JRE or JDK Version: jdk 11
• OpenRefine: 3.6

Datasets

Extract:
{
"version": "1.0",
"mediawiki": {
"name": "Wikimedia Commons",
"root": "https://commons.wikimedia.org/wiki",
"main_page": "https://commons.wikimedia.org/wiki/Main_Page",
"api": "https://commons.wikimedia.org/w/api.php"
},
//..
"wikibase": {
"site_iri": "https://commons.wikimedia.org/entity",
"maxlag": 5,
"properties": {
"instance_of": "P31",
"subclass_of": "P279",
}
}

Additional context

n/a

[wetneb]

Closed by #4341.