fixed: FSharpEntity.TryFullName raises exception on erased types

fixed: Checker.GetToolTipTextAlternate raises exception on outdated assemblies

Fix https://github.com/fsprojects/VisualFSharpPowerTools/issues/707, https://github.com/fsprojects/VisualFSharpPowerTools/issues/706
Isn't `String.Join` already inside `Option.attempt`?..