Avoid wildcard except at the top level of the call stack

[etingof]

The main I/O loops tend to catch all exceptions coming deep from the call stack and re-raise them as PySnmpError. As noted in #167, this prevents user callback from raising their own exceptions and making their way up to the main loop.

The current design should probably be changed in a way that pysnmp code should so this exception catching closer to the actual call of an underlying library (and re-raise if needed). But at the top level of the main loop no exceptions should be caught.