Improve Error Message when Extension Category does not exist

[benibenj]

When attempting to publish an extension with an unsupported category through vsce or the Visual Studio Marketplace web UI, users encounter a non-descriptive error message that leads to confusion and unnecessary issues being filed.

Current Behavior:
The current error message displayed is:

• Error: Specified argument was out of the range of valid values. (Parameter 'start')

This message does not clearly indicate the actual problem, resulting in users filing issues against vsce (e.g., vscode-vsce#977) and the Marketplace (e.g., vsmarketplace#595).

Proposed Solution:
A more informative error message would significantly improve the user experience. For example:

• Error: The provided category 'CATEGORY_NAME' is not supported. Please refer to the following documentation for a list of supported categories: LINK_TO_DOCS

This change would help users self-diagnose the issue and take corrective action without the need to file additional support issues.