Add custom errors

src/errors.js

@@ -0,0 +1,41 @@
+Error.subclass = function(errorName) {
+  var newError = function(message, data) {
+    this.name    = errorName;
+    this.message = (message || "");
+    this.data = data;
+  };
+
+  newError.subclass = this.subclass;
+  inherits(newError, this);
+
+  return newError;
+};
+
+export var NetworkError = Error.subclass("NetworkError");
+export var NotFoundError = NetworkError.subclass("NotFoundError");
+
+/**
+ * From: https://github.com/joyent/node/blob/master/lib/util.js
+ * Inherit the prototype methods from one constructor into another.
+ *
+ * The Function.prototype.inherits from lang.js rewritten as a standalone
+ * function (not on Function.prototype). NOTE: If this file is to be loaded
+ * during bootstrapping this function needs to be rewritten using some native
+ * functions as prototype setup using normal JavaScript does not work as
+ * expected during bootstrapping (see mirror.js in r114903).
+ *
+ * @param {function} ctor Constructor function which needs to inherit the
+ *     prototype.
+ * @param {function} superCtor Constructor function to inherit prototype from.
+ */
+function inherits(ctor, superCtor) {
+  ctor.super_ = superCtor;
+  ctor.prototype = Object.create(superCtor.prototype, {
+    constructor: {
+      value: ctor,
+      enumerable: false,
+      writable: true,
+      configurable: true
+    }
+  });
+}

src/index.js

@@ -6,4 +6,5 @@ export {Server} from "./server";
 export {Operation} from "./operation";
 export {Keypair} from "stellar-base";
 export {Memo} from "./memo";
-export {xdr} from "stellar-base";
+export {xdr} from "stellar-base";
+export * from "./errors";