Improve ARM error parsing and display (#189)

src/Utils/arm/request.ts

@@ -6,11 +6,30 @@ Instead, generate ARM clients that consume this function with stricter typing.
 */
 
 import promiseRetry, { AbortError } from "p-retry";
-import { ErrorResponse } from "./generatedClients/2020-04-01/types";
 import { userContext } from "../../UserContext";
 
-interface ARMError extends Error {
+interface ErrorResponse {
   code: string;
+  message: string;
+}
+
+// ARM sometimes returns an error wrapped in a top level error object
+// Example: 409 Conflict error when trying to delete a locked resource
+interface WrappedErrorResponse {
+  error: ErrorResponse;
+}
+
+type ParsedErrorResponse = ErrorResponse | WrappedErrorResponse;
+
+export class ARMError extends Error {
+  constructor(message: string) {
+    super(message);
+    // Set the prototype explicitly.
+    // https://github.com/Microsoft/TypeScript/wiki/FAQ#why-doesnt-extending-built-ins-like-error-array-and-map-work
+    Object.setPrototypeOf(this, ARMError.prototype);
+  }
+
+  public code: string | number;
 }
 
 interface Options {
@@ -33,9 +52,20 @@ export async function armRequest<T>({ host, path, apiVersion, method, body: requ
     body: requestBody ? JSON.stringify(requestBody) : undefined
   });
   if (!response.ok) {
-    const errorResponse = (await response.json()) as ErrorResponse;
-    const error = new Error(errorResponse.message) as ARMError;
-    error.code = errorResponse.code;
+    let error: ARMError;
+    try {
+      const errorResponse = (await response.json()) as ParsedErrorResponse;
+      if ("error" in errorResponse) {
+        error = new ARMError(errorResponse.error.message);
+        error.code = errorResponse.error.code;
+      } else {
+        error = new ARMError(errorResponse.message);
+        error.code = errorResponse.code;
+      }
+    } catch (error) {
+      throw new Error(await response.text());
+    }
+
     throw error;
   }