Bug/schemacompare publish and script tasks to send back results (#830)

* Fix Schema compare Publish and Script generation task to return back error messages correctly.

* Elaborating test a bit more to cover tasks

* Validate one task at a time for better clarity

* send back only errors not whole messages since task view doesnt have a good way (other than hover text) to show long messages

* Fixing the negative test cases

src/Microsoft.SqlTools.ServiceLayer/SchemaCompare/SchemaCompareGenerateScriptOperation.cs

@@ -69,6 +69,11 @@ namespace Microsoft.SqlTools.ServiceLayer.SchemaCompare
                         this.SqlTask.AddScript(SqlTaskStatus.Succeeded, ScriptGenerationResult.MasterScript);
                     }
                 }
+                if (!this.ScriptGenerationResult.Success)
+                {
+                    ErrorMessage = this.ScriptGenerationResult.Message;
+                    throw new Exception(ErrorMessage);
+                }
             }
             catch (Exception e)
             {