Added errors for providers trying to use ENS on networks without ENS.

Author: ricmoo

providers/package.json

@@ -1,6 +1,6 @@
 {
   "name": "ethers-providers",
-  "version": "2.1.15",
+  "version": "2.1.16",
   "description": "Service provider for Ethereum wallet library.",
   "bugs": {
     "url": "http://github.com/ethers-io/ethers.js/issues",

providers/provider.js

@@ -845,6 +845,8 @@ utils.defineProperty(Provider.prototype, 'resolveName', function(name) {
         return Promise.resolve(utils.getAddress(name));
     } catch (error) { }
 
+    if (!this.ensAddress) { throw new Error('network does not have ENS deployed'); }
+
     var self = this;
 
     var nodeHash = utils.namehash(name);
@@ -867,6 +869,8 @@ utils.defineProperty(Provider.prototype, 'resolveName', function(name) {
 });
 
 utils.defineProperty(Provider.prototype, 'lookupAddress', function(address) {
+    if (!this.ensAddress) { throw new Error('network does not have ENS deployed'); }
+
     address = utils.getAddress(address);
 
     var name = address.substring(2) + '.addr.reverse'