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