IsValidExtensionAgent could show validation error:

[jessehouwing]

Looks like a validation error is actually returned when querying the status: validationResultMessage

Example

2018-04-14T10:43:30.7515913Z GET https://marketplace.visualstudio.com/_apis/gallery/publishers/agile-extensions/extensions/bravo-notes-staging?flags=1
                Response Code: OK
                Response: {"publisher":{"publisherId":"c598cbbe-5149-476f-ad05-e1573099cd8f","publisherName":"agile-extensions","displayName":"Agile Extensions","flags":"verified"},"extensionId":"b98fb4ac-8523-446a-a26e-3213351df887","extensionName":"bravo-notes-staging","displayName":"Bravo Notes (Staging)","flags":"validated","lastUpdated":"2018-04-14T10:42:36.137Z","publishedDate":"2018-03-13T09:28:15.61Z","shortDescription":"Release notes without the pain","versions":[{"version":"3.0.630","flags":"validated","lastUpdated":"2018-04-02T06:39:45.483Z"},{"version":"3.0.629","flags":"none","lastUpdated":"2018-04-02T06:31:25.243Z","validationResultMessage":"Following errors were found in endpoint contribution:\r\n      Authentication schemes endpoint-auth-scheme-basic, are not valid."},{"version":"3.0.628","flags":"none","lastUpdated":"2018-04-02T06:22:07.877Z","validationResultMessage":"EndpointId is missing or empty in dataSourceBinding of task with ID 'ec0ce74c-0b96-4770-9bfd-609f6ae94cec'."},"deploymentType":0}

It probably makes sense to render the error as part of the failure message.