Idea: tt start should check instance start status

[psergee]

tt start should include a sanity check to ensure that the Tarantool instances are starting correctly. Sometimes, an instance may exit with an error for some reason, but tt start does not report this, so tt start now effectively consists of two commands: tt start + tt status. tt status should be used to check whether all instances have started. tt start could wait for a short period of time to check if the instances are still running and, if not, it could print the exit code and the last log lines, for example.