FIX for application restart when srv not reachable

[v0lkan]

Tested on Mac OS X Yosemite, Chrome Version 43.0.2357.132 (64-bit)

Steps to reproduce:

1. Run service by `dashing start`
2. View a dashboard
3. kill the service by CTRL + D
4. wait for five minutes.

expectation:
page should reload

what happens:
e.currentTarget.readyState is  0 (not initialized)
chrome's EventSource API tries to reconnect to a non-existent server over and over again.
The page never refreshes.

We clear errorTimerId immediately after we get a message event, so that there won't be redundant restarts.

On an edge case, the service might reconnect and won't get any messages for a while which might trigger the timer with errorTimerId; and that will cause only one extra refresh only.