Project references

• Get the GetDocumentInsider to spit out errors to StdErr
  • For case where no documents are registered (or the service isn't registered), move messages from the targets file into the tool
• Determine why insider is spitting out "Generating document named 'v1'" after an error and fix that confusing output
• Apply existing multi-targeting TFM handling for OpenApiGetDocuments target to the GenerateOpenApiDocuments
  • Add errors if only .NET Core 2.0 or less is targeted (feature should be disabled in that case)

File references

• Add client error to detect when JSON file doesn't exist
• Remove IgnoreExitCode from NSwag targets (in NSwag repo)