* Fix#4029 Ensure changeKernels always resolves, even in error states
This is necessary to unblock reverting the kernel on canceling Python install
- startSession now correctly sets up kernel information, since a kernel is loaded there.
- Remove call to change kernel on session initialize. This isn't needed due to refactor
- Handle kernel change failure by attempting to fall back to old kernel
- ExtensionHost $startNewSession now ensures errors are sent across the wire.
- Update AttachTo and Kernel dropdowns so they handle kernel being available. This is needed since other changes mean the session is likely ready before these get going
* Fix to handle python cancel and load existing scenarios
- Made changes to handle failure flow when Python dialog is canceled
- Made changes to handle initial load fail. Kernel and Attach To dropdowns show No Kernel / None and you can choose a kernel
- Added error wrapping in ext host so that string errors make it across and aren't lost.
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