`tap package download` Add a more user friendly message when downloading an incompatible package

Environment:

Windows 10, 64-bit

OpenTAP 9.14.0-rc.1 (reproduced 9.13)

Steps to reproduce:

1.tap package download RemoteSigntool -r "http://packages.opentap.keysight.com"

Actual result:

"A CliAction has thrown an exception: Object reference not set to an instance of an object."

Expected result:

A more user friendly message

Other remarks:

Also tap package show "Results Viewer CE" --os Linux