[web tests] A more informative error message

When run_web_tests.py --driver-name=... is used, print the driver name
on error instead of the default driver name.

Bug: None
Change-Id: I5577fe5e774f6fd9959737d3e2792c20e5fc9359
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341649Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796039}

third_party/blink/tools/blinkpy/web_tests/port/driver.py

@@ -451,7 +451,7 @@ class Driver(object):
             deadline = time.time() + DRIVER_START_TIMEOUT_SECS
             if not self._wait_for_server_process_output(
                     self._server_process, deadline, '#READY'):
-                _log.error('content_shell took too long to startup.')
+                _log.error('%s took too long to startup.' % server_name)
 
     def _wait_for_server_process_output(self, server_process, deadline, text):
         output = ''