Extension bugged when using an unsupported default region

## Problem
When using an unsupported default region, the extension loads with an empty list of regions. Its impossible to add any region but there are no visible errors or logs to understand what's going on.

## Steps to reproduce the issue

1. Configure aws cli with a default profile that uses an unsupported aws toolkit region (i.e: me-central-1)
2. Open VsCode and install AWS toolkit extension
3. Open the extension Panel, observe an empty list of regions listed.
4. Click on Add regions to AWS Explorer...
5. Observe that no regions are clickable/available

## Expected behavior
Advise the user that the region configured is not supported and offer to select another.

## System details (run the `AWS: About Toolkit` command)

- OS: Windows_NT x64 10.0.19044
- Visual Studio Code version: 1.74.2
- AWS Toolkit version: 1.60.0

Start by tracing how the AWS Toolkit reads the default profile region when the extension panel opens, then follow the Add regions to AWS Explorer flow. Reproduce with an unsupported region such as me-central-1 and verify that the user receives a visible explanation and can select another supported region.

Written by the indexing model from the issue text.