Export errors to allow typed access to them

Exports the errors module and the types it contains as `stripe.errors`.
This allows users to match on an error with the `instanceof` operator
instead of having to match on the string generated by accessing an
object's `type`.

Fixes #457.

lib/stripe.js

@@ -107,6 +107,7 @@ function Stripe(key, version) {
   this.setApiKey(key);
   this.setApiVersion(version);
 
+  this.errors = require('./Error.js');
   this.webhooks = require('./Webhooks');
 }
 

test/flows.spec.js

@@ -515,5 +515,11 @@ describe('Flows', function() {
         done();
       });
     });
+
+    it('Exports errors as types', function() {
+      expect(new stripe.errors.StripeInvalidRequestError({
+        message: 'error'
+      }).type).to.equal('StripeInvalidRequestError');
+    });
   });
 });