show connection failure error message in admin form

Problem/Motivation

When the connection to Unomi API fails, the form just says:

> 'Connection failed, there was a problem with the connection to the UNOMI API instance'

This is not very helpful, as there's no indication of what the problem is.

Steps to reproduce

Proposed resolution

Show the exception message in the form message.

Remaining tasks

User interface changes

API changes

Data model changes