properly handle import errors in the isolated capacity healthcheck

if the awx_capacity module runs on an isolated node with missing
libraries (i.e., psutil) or bad permissions, then the runner status will
be "failed"

in this scenario, we *still* want to react by recording a capacity=0

awx/main/isolated/manager.py

@@ -370,7 +370,6 @@ class IsolatedManager(object):
                 private_data_dir
             )
 
-            if runner_obj.status == 'successful':
             for instance in instance_qs:
                 task_result = {}
                 try: