stop port agent even if there's an exception while stopping the instrument agent instance

Strategy implemented while investigating the issue of failures while stopping instrument agents, see:

http://buildbot.oceanobservatories.org:8010/builders/coi_nightly/builds/446/steps/sa/logs/stdio

Except for the first one, all the platform tests involving instruments are failing to launch the corresp port agent, and that's because the stopping of the instrument agent in the first test failed.  The change is to stop the port agent even if there's an exception while stopping the instrument agent.

Local testing OK, that is, all platform agent test with instruments are passing, but again, the stop of the instrument agents  at the end of each test is still failing.