Fixed: Unhelpful error message when PhantomJS exits.

Selenium does not check that the PhantomJS process is running prior to
checking connectivity. This results in unhelpful error message when the
subprocess unexpectedly exits.

The solution is to assert that the process is still running before
connectivity is checked.

Fixes SeleniumHQ#2168.

py/selenium/webdriver/common/service.py

@@ -81,12 +81,23 @@ def start(self):
                 (os.path.basename(self.path), self.start_error_message, str(e))
                 )
         count = 0
-        while not self.is_connectable():
+        while True:
+            self.assert_process_still_running()
+            if self.is_connectable():
+                break
             count += 1
             time.sleep(1)
             if count == 30:
                 raise WebDriverException("Can not connect to the Service %s" % self.path)
 
+    def assert_process_still_running(self):
+        return_code = self.process.poll()
+        if return_code is not None:
+            raise WebDriverException(
+                'Service %s unexpectedly exited. Status code was: %s'
+                % (self.path, return_code)
+            )
+
     def is_connectable(self):
         return utils.is_connectable(self.port)