[BugFix] On browserRelogin, clear timeouts if socket is not null & log.warn instead of error for reconnection from unknown browser

[step2yeung]

Problem:

1. On browserRelogin event, there are times where the socket is not null. When we call tryAttach() on an existing socket, eventListeners attached from tryAttach() will be duplicated.

2. When there is an unknown browser reconnecting, app.js will throw an error and the run will fail. This could happen if a previous run's browser for some reason did not terminate.

Solutions:

1. On browserRelogin, clear timeouts if socket is not null. This will prevent socket events duplication.

2. log.warn instead of throwing an error for reconnection from unknown browser. This way, the current run won't be punished for dangling browsers from previous runs.

[davecombs]

I am really happy to see this fix! Thank you, Stephen! Looks good to me.

[step2yeung]

Travis failure due to sauce lab issues: #1352

[step2yeung]

@stefanpenner Would love to get your eyes on this PR too!

[stefanpenner]

@step2yeung LGTM –