run-webkit-tests: fix crash when trying to read a log that did not exist.

During error recovery after failing to start a server, there might actually
not be an error log where we think there should be one.

TBR=eseidel@chromium.org
BUG=378444

Review URL: https://codereview.chromium.org/421413002

git-svn-id: svn://svn.chromium.org/blink/trunk@179077 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/servers/server_base.py

@@ -210,7 +210,7 @@ class ServerBase(object):
                 _log.error('%s no stderr handle' % self._name)
         else:
             _log.error('%s no process' % self._name)
-        if self._error_log_path:
+        if self._error_log_path and self._filesystem.exists(self._error_log_path):
             error_log_text = self._filesystem.read_text_file(self._error_log_path)
             if error_log_text:
                 _log.error('%s error log (%s) contents:' % (self._name, self._error_log_path))