usdt.py: improve error messags for enable_probe()
usdt.py: improve error messags for enable_probe()

iovisor/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc