rconn: Discover errors in rconn_run() even if rconn_recv() is never called.

rconn: Discover errors in rconn_run() even if rconn_recv() is never called.

rconn_recv() calls vconn_recv(), which will report a connection error and
cause rconn_recv() to disconnect the rconn.  Most rconn users regularly
call rconn_recv(), so that disconnection happens promptly.  However,
the lswitch code only calls rconn_recv() after the connection negotiates an
OpenFlow version, so a connection that failed before negotiation would
never be detected as failed.  This commit fixes the problem by making
rconn_run() also detect and handle connection errors.

The lswitch code is only used by the test-controller program, so this is
not an important bug fix.

Reported-by: Vasu Dasari <vdasari@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>