parse gallery error message

fixes #6761

src/vs/code/node/cliProcessMain.ts

@@ -15,7 +15,7 @@ import { IEnvironmentService } from 'vs/platform/environment/common/environment'
 import { EnvironmentService } from 'vs/platform/environment/node/environmentService';
 import { IEventService } from 'vs/platform/event/common/event';
 import { EventService } from 'vs/platform/event/common/eventService';
-import { IExtensionManagementService, IExtensionGalleryService } from 'vs/platform/extensionManagement/common/extensionManagement';
+import { IExtensionManagementService, IExtensionGalleryService, IQueryResult } from 'vs/platform/extensionManagement/common/extensionManagement';
 import { getExtensionId } from 'vs/platform/extensionManagement/node/extensionManagementUtil';
 import { ExtensionManagementService } from 'vs/platform/extensionManagement/node/extensionManagementService';
 import { ExtensionGalleryService } from 'vs/platform/extensionManagement/node/extensionGalleryService';
@@ -68,7 +68,18 @@ class Main {
 					return TPromise.wrapError(localize('alreadyInstalled', "Extension '{0}' is already installed.", id));
 				}
 
-				return this.extensionGalleryService.query({ ids: [id] }).then(result => {
+				return this.extensionGalleryService.query({ ids: [id] })
+					.then<IQueryResult>(null, err => {
+						if (err.responseText) {
+							try {
+								const response = JSON.parse(err.responseText);
+								return TPromise.wrapError(response.message);
+							} catch (e) {
+								return TPromise.wrapError(err);
+							}
+						}
+					})
+					.then(result => {
 						const [extension] = result.firstPage;
 
 						if (!extension) {