fix: handle failure to retrieve net_version when forking

lib/forking/forked_blockchain.js

@@ -171,9 +171,13 @@ ForkedBlockchain.prototype.initialize = async function(accounts, callback) {
     const forkVersion = await new Promise((resolve, reject) => {
       this.web3.eth.net.getId((err, version) => {
         if (err) {
-          Error.captureStackTrace(err);
-          err.message = `The fork provider errored when checking net_version: ${err.message}`;
-          reject(err);
+          if (this.options.network_id) {
+            resolve(this.options.network_id);
+          } else {
+            Error.captureStackTrace(err);
+            err.message = `The fork provider errored when checking net_version: ${err.message}`;
+            reject(err);
+          }
         } else {
           resolve(version);
         }