Update mimetype parse error message

Fix #130002

src/vs/workbench/api/common/extHostTypes.ts

@@ -3107,7 +3107,7 @@ export class NotebookCellOutputItem {
 	) {
 		const mimeNormalized = normalizeMimeType(mime, true);
 		if (!mimeNormalized) {
-			throw new Error('INVALID mime type, must not be empty or falsy: ' + mime);
+			throw new Error(`INVALID mime type: ${mime}. Must be in the format "type/subtype[;optionalparameter]"`);
 		}
 		this.mime = mimeNormalized;
 	}