Server#responsive? now rescues all SystemCallError exceptions

It previously only rescued Errno::ECONNREFUSED and Errno::EBADF. On a
few occasions, I have also seen the Errno::ETIMEDOUT exception raised by
Net::HTTP from this method. Since the Net::HTTP calls are the only thing
in this method that should be making any kind of system calls, a rescue
of all SystemCallError exceptions (the superclass of all of the Errno
exceptions) will catch these as well as any other system call errors
triggered by Net::HTTP.