Log WorkspaceFail events (#1738)

MSBuildWorkspace.OpenSolutionAsync() and MSBuildWorkspace.OpenProjectAsync() do not give any notification that loading a solution or project failed. However, there is a WorkspaceFailed event that we can use to be notified when there is a problem. This logs the diagnotic information from this event to help the user troubleshoot.

Related:
* dotnet/roslyn#19978 (comment)
* #1686
* #1708 (comment)

src/Microsoft.DocAsCode.Metadata.ManagedReference/ExtractMetadataWorker.cs

@@ -71,7 +71,15 @@ public ExtractMetadataWorker(ExtractMetadataInputModel input, bool rebuild, bool
  _msbuildProperties["Configuration"] = "Release";
  }
 
- _workspace = new Lazy<MSBuildWorkspace>(() => MSBuildWorkspace.Create(_msbuildProperties));
+ _workspace = new Lazy<MSBuildWorkspace>(() =>
+ {
+ var workspace = MSBuildWorkspace.Create(_msbuildProperties);
+ workspace.WorkspaceFailed += (s, e) =>
+ {
+ Logger.LogWarning($"Workspace failed with: {e.Diagnostic}");
+ };
+ return workspace;
+ });
  }
 
  public async Task ExtractMetadataAsync()