Allow to use BrowserStackTunnel without defined servers

by ignoring the result of `BrowserStackLocal` binary, because a tunnel to BrowserStack just can be opened when at least one server was defined (I've located this issue after I've tried to invoke the BrowserStackTunnel without any server and it failed with `Error: Atleast two arguments are needed!`).

This pull request also documents that BrowserStack requires that each private or local server, that should be accessed by BrowserStack, needs to be listed within `servers`.

CLA is on file.