Added toJSON to decorated Axios errors to faciliate serialization (#1625)

lib/core/enhanceError.js

@@ -17,5 +17,23 @@ module.exports = function enhanceError(error, config, code, request, response) {
   }
   error.request = request;
   error.response = response;
+  error.toJSON = function() {
+    return {
+      // Standard
+      message: this.message,
+      name: this.name,
+      // Microsoft
+      description: this.description,
+      number: this.number,
+      // Mozilla
+      fileName: this.fileName,
+      lineNumber: this.lineNumber,
+      columnNumber: this.columnNumber,
+      stack: this.stack,
+      // Axios
+      config: this.config,
+      code: this.code
+    };
+  };
   return error;
 };

test/specs/core/createError.spec.js

@@ -12,4 +12,17 @@ describe('core::createError', function() {
     expect(error.request).toBe(request);
     expect(error.response).toBe(response);
   });
+  it('should create an Error that can be serialized to JSON', function() {
+    // Attempting to serialize request and response results in
+    //    TypeError: Converting circular structure to JSON
+    var request = { path: '/foo' };
+    var response = { status: 200, data: { foo: 'bar' } };
+    var error = createError('Boom!', { foo: 'bar' }, 'ESOMETHING', request, response);
+    var json = error.toJSON();
+    expect(json.message).toBe('Boom!');
+    expect(json.config).toEqual({ foo: 'bar' });
+    expect(json.code).toBe('ESOMETHING');
+    expect(json.request).toBe(undefined);
+    expect(json.response).toBe(undefined);
+  });
 });