crashes: simplify error handling in send_pkt_out
crashes: less log flushing

Consistent updates flushes the log a lot. Remove the flush calls from
the per-switch operations but leave the top-level flush calls in the
implement_policy function so that the log does get flushed before each
stage of the consistent update process.
crashes: handle failures in PerPacketConsistent.bring_up_switch

By the time a SwitchUp event gets handled, the switch may have already
disconnected. Instead of crashing, print out the error and continue on.
crashes: consistent updates can handle a switch disconnect