gray out nonexistent multi root folder

fixes #29738

src/vs/workbench/parts/files/browser/media/explorerviewlet.css

@@ -142,6 +142,10 @@
 	padding: 0 20px 0 20px;
 }
 
+.explorer-viewlet .explorer-item.nonexistent-root {
+	opacity: 0.5;
+}
+
 .explorer-viewlet .explorer-item .monaco-inputbox {
 	width: 100%;
 	line-height: normal;

src/vs/workbench/parts/files/browser/views/explorerViewer.ts

@@ -106,7 +106,14 @@ export class FileDataSource implements IDataSource {
 
 				return stat.children;
 			}, (e: any) => {
-				this.messageService.show(Severity.Error, e);
+				stat.exists = false;
+				stat.hasChildren = false;
+				if (!stat.isRoot) {
+					this.messageService.show(Severity.Error, e);
+				} else {
+					// We render the roots that do not exist differently, nned to do a refresh
+					tree.refresh(stat, false);
+				}
 
 				return []; // we could not resolve any children because of an error
 			});
@@ -311,6 +318,9 @@ export class FileRenderer implements IRenderer {
 		if (!editableData) {
 			templateData.label.element.style.display = 'block';
 			const extraClasses = ['explorer-item'];
+			if (!stat.exists && stat.isRoot) {
+				extraClasses.push('nonexistent-root');
+			}
 			templateData.label.setFile(stat.resource, { hidePath: true, fileKind: stat.isRoot ? FileKind.ROOT_FOLDER : stat.isDirectory ? FileKind.FOLDER : FileKind.FILE, extraClasses });
 		}
 

src/vs/workbench/parts/files/common/explorerModel.ts

@@ -71,6 +71,7 @@ export class FileStat implements IFileStat {
 	public children: FileStat[];
 	public parent: FileStat;
 
+	public exists: boolean;
 	public isDirectoryResolved: boolean;
 
 	constructor(resource: URI, public root: FileStat, isDirectory?: boolean, hasChildren?: boolean, name: string = paths.basename(resource.fsPath), mtime?: number, etag?: string) {
@@ -90,6 +91,7 @@ export class FileStat implements IFileStat {
 		}
 
 		this.isDirectoryResolved = false;
+		this.exists = true;
 	}
 
 	public getId(): string {