Raise better errors if server fails to start

* When WEBKIT_SERVER_START_TIMEOUT was reached, @port would get set to
  nil. If execution of the SERVER_PROCESS returned immediately, @port
  would get set to 0. This lead to odd cross-platform behavior in
  #connect, which would try to connect to a nonsense port.
Merge branch 'master' of github.com:thoughtbot/capybara-webkit

* 'master' of github.com:thoughtbot/capybara-webkit:
  Raise better errors if server fails to start