Catch broad exception in methods used in FixedIntervalLoopingCall

Catch broad exception in methods used in FixedIntervalLoopingCall

Unlike other places where it might make sense to catch specific
exceptions, methods that are used to check L3 and DHCP agents
liveness via FixedIntervalLoopingCall should never allow exceptions
to leak to calling method and interrupt the loop.

Further improvement of FixedIntervalLoopingCall might be needed,
but for the sake of easy backporting it makes sense to fix the issue
in neutron before pushing refactoring to 3rd-party library.

Change-Id: I6a61e99a6f4e445e26ea4a9923b47e35559e5703
Closes-Bug: #1458119