Add LanguageFlavorNotification handling and refactor LanguageService (#375)

* Add LanguageFlavorNotification handling and refactor LanguageService
- Added new notification handler for language flavor changed
- Refactored the LanguageService so that it no longer relies on so many intertwined static calls, which meant it was impossible to test without modifying the static instance. This will help with test reliability in the future, and prep for replacing the instance with a service provider.

* Skip if not an MSSQL doc and add test

* Handle definition requests

* Fix diagnostics handling

src/Microsoft.SqlTools.ServiceLayer/ObjectExplorer/SmoModel/DatabaseTreeNode.cs

@@ -47,7 +47,11 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.SmoModel
             }
             else
             {
-                ErrorMessage = string.Format(CultureInfo.InvariantCulture, SR.DatabaseNotAccessible, context.Database.Name);
+                if (string.IsNullOrEmpty(ErrorMessage))
+                {
+                    // Write error message if it wasn't already set during IsAccessible check
+                    ErrorMessage = string.Format(CultureInfo.InvariantCulture, SR.DatabaseNotAccessible, context.Database.Name);
+                }
             }
         }
 
@@ -63,11 +67,11 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.SmoModel
             }
             catch (Exception ex)
             {
-                return true;
                 string error = string.Format(CultureInfo.InvariantCulture, "Failed to get IsAccessible. error:{0} inner:{1} stacktrace:{2}",
                     ex.Message, ex.InnerException != null ? ex.InnerException.Message : "", ex.StackTrace);
                 Logger.Write(LogLevel.Error, error);
                 ErrorMessage = ex.Message;
+                return false;
             }
         }
     }