[js] Handle rejection of the driver

If binding can't find the executable we need to handle the rejection not
just the exit of the process

Author: AutomatedTester

javascript/node/selenium-webdriver/io/exec.js

@@ -134,12 +134,16 @@ function exec(command, opt_options) {
   proc.unref()
   process.once('exit', onProcessExit)
 
-  const result = new Promise((resolve) => {
+  const result = new Promise((resolve, reject) => {
     proc.once('exit', (code, signal) => {
       proc = null
       process.removeListener('exit', onProcessExit)
       resolve(new Result(code, signal))
     })
+
+    proc.once('error', err => {
+      reject(err);
+    })
   })
   return new Command(result, killCommand)