usrp2: try/catch for first send of discovery routine

This send can fail with a network down/unreachable error.
The try/catch allows for discovery to continue.
utils: usrp_n2xx_net_burner_gui no longer ignores dont-check-rev option
uhd: fix get_xx_freq_range to use analog bw/2

The proper equation should be:

max_freq = max_lo + min(adc/2, bw/2) - sr/2