[Debugger] Tooltips should not be shown for conditional method invocations

When hovering the mouse cursor over most method invocation
expressions, the logic returns a null expression to evaluate
(thereby bypassing evaluation of said expression), but when
hovering over conditional method invocations, the logic failed.

Since the logic is identical to VS Windows, do what VS Windows does
in this scenario as well, which is to ignore evaluation errors
and not show them.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1054139

Author: jstedfast

main/src/addins/MonoDevelop.Debugger/MonoDevelop.Debugger.VSTextView/QuickInfo/DebuggerQuickInfoSource.cs

@@ -135,7 +135,7 @@ async Task EvaluateAndShowTooltipAsync (IAsyncQuickInfoSession session, ITextVie
 				if (cancellationToken.IsCancellationRequested)
 					return;
 
-				if (val == null || val.IsUnknown || val.IsNotSupported)
+				if (val == null || val.IsUnknown || val.IsNotSupported || val.IsError)
 					return;
 
 				timer.Success = true;