Fix broken exception handler when adding USM users to SNMP session

[lunkwill42]

This line is broken:

pynetsnmp/pynetsnmp/netsnmp.py

Line 635 in abf7297

except StandardError as e:

There is no such exception class, and it's not entirely clear from the git blame output why this was put in there, or how it was supposed to work at the time it was added.