Concat uiSideErrors in Cell output (#11364)

* Concat uiSideErrors in Cell output
* Better key

Author: DonJayamanne

src/datascience-ui/interactive-common/cellOutput.tsx

@@ -200,6 +200,9 @@ export class CellOutput extends React.Component<ICellOutputProps> {
         if (nextProps.cellVM.cell.data.outputs !== this.props.cellVM.cell.data.outputs) {
             return true;
         }
+        if (nextProps.cellVM.uiSideError !== this.props.cellVM.uiSideError) {
+            return true;
+        }
         if (
             !this.isCodeCell() &&
             nextProps.cellVM.cell.id !== Identifiers.EditCellId &&
@@ -272,7 +275,7 @@ export class CellOutput extends React.Component<ICellOutputProps> {
             // tslint:disable: react-no-dangerous-html
             if (this.props.cellVM.uiSideError) {
                 outputs.push(
-                    <div className="cell-output-uiSideError">
+                    <div key={'uiError'} className="cell-output-uiSideError">
                         <div dangerouslySetInnerHTML={{ __html: this.props.cellVM.uiSideError }} />
                     </div>
                 );

src/datascience-ui/interactive-common/redux/reducers/commonEffects.ts

@@ -243,9 +243,11 @@ export namespace CommonEffects {
                     "Widgets require us to download supporting files from a 3rd party website. Click <a href='https://command:python.datascience.enableLoadingWidgetScriptsFromThirdPartySource'>here</a> to enable this or click <a href='https://aka.ms/PVSCIPyWidgets'>here</a> for more information. (Error loading {0}:{1})."
                 ).format(arg.payload.data.moduleName, arg.payload.data.moduleVersion);
             }
+            // Preserve existing error messages.
+            const existingErrorMessage = current.uiSideError ? `${current.uiSideError}\n` : '';
             newVMs[index] = Helpers.asCellViewModel({
                 ...current,
-                uiSideError: errorMessage
+                uiSideError: `${existingErrorMessage}${errorMessage}`
             });
 
             // Make sure to tell the extension so it can log telemetry.