fix: handle missing errorIndication in StatusInformation

[saschabuehrle]

Fixes #241

rfc3412.prepare_data_elements can raise a StatusInformation with only sendPduHandle and no errorIndication key (line 875 in rfc3412.py, the else-branch when reportableFlag is false). When cmdgen.process_response_pdu catches this, it does statusInformation["errorIndication"] which throws a KeyError.

Switched both lookups in cmdgen.py to .get("errorIndication", errind.requestTimedOut) so the callback still fires with a sensible error instead of crashing.

-- saschabuehrle

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