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fixed: Checker.GetToolTipTextAlternate raises exception on outdated assemblies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
Perhaps moving `String.Join` part into `Option.attempt` closure. I'm not confident that `x,AccessPath` will never throw exceptions.