Improve error handling for `idaes get-extensions`

[lbianchi-lbl]

Problem

Misleading error message is displayed if idaes get-extensions fails for spurious reasons

Possible solution

• Improve error handling so that more specific exceptions are raised and displayed to the user
• Create dedicated exception subclass (e.g. UnsupportedPlatform) and only raise that when appropriate
  • e.g. if detected platform is not found after applying compatibility mapping
  • Possibly before a download is even attempted?