[BUG] some servicemon checker runs crash with a UnboundLocalError

[lunkwill42]

Describe the bug

Once #2057 has been applied, some servicemon checker runs crash with a strange UnboundLocalError

Expected behavior

No unhandled exceptions should leak through plugin runs.

Environment (please complete the following information):

• NAV version installed: 5.0.2

Traceback

Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/statemon/RunQueue.py", line 67, in run
    self.execute(checker)
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/statemon/RunQueue.py", line 80, in execute
    checker.run()
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/statemon/abstractchecker.py", line 109, in run
    status, info = self.execute_test()
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/statemon/abstractchecker.py", line 194, in execute_test
    return status, info
UnboundLocalError: local variable 'info' referenced before assignment

Additional context

The problem appears to stem from a change in the way except clauses are handled in Python 3 vs Python 2. The exception handler in the affected code reuses the exception variable in the outer scope, but this no longer works in Python 3.