Do not leak cancellation exception for our own operation

[CyrusNajmabadi]

Backport #78376 to dev17.14

[CyrusNajmabadi]

@arunchndr ptal.

[arunchndr]

@arunchndr ptal.

the NRE looks legit?

[CyrusNajmabadi]

@arunchndr the crash is in navbars, which i'm not touching. I think it's possibly in the test code that is trying to hook up navbars. i.e.:

   at Microsoft.CodeAnalysis.UnitTests.ObjectExtensions.GetFieldValue(Object instance, String fieldName) in /_/src/Workspaces/CoreTestUtilities/ObjectExtensions.cs:line 49
   at Microsoft.CodeAnalysis.UnitTests.ObjectExtensions.GetFieldValue[FieldType](Object instance, String fieldName) in /_/src/Workspaces/CoreTestUtilities/ObjectExtensions.cs:line 58
   at Microsoft.VisualStudio.Extensibility.Testing.EditorInProcess.<GetNavigationBarComboBoxesAsync>d__39.MoveNext() in /_/src/VisualStudio/IntegrationTest/New.IntegrationTests/InProcess/EditorInProcess.cs:line 569

[JoeRobich]

May be fixed by @jasonmalinowski's backport #79076

[jasonmalinowski]

Yep that was the issue I fixed.

[CyrusNajmabadi]

@arunchndr this si green