Fix test_query_concurrency exception handling.
Fix test_query_concurrency exception handling.

Fixes use of an undefined variable.

I saw the following message in a build failure, which
clearly wasn't intended:

  MainThread: Debug webpage not yet available.
  Exception in thread Thread-862:
  Traceback (most recent call last):
    File "/usr/lib64/python2.7/threading.py", line 811, in __bootstrap_inner
      self.run()
    File "/usr/lib64/python2.7/threading.py", line 764, in run
      self.__target(*self.__args, **self.__kwargs)
    File "/data/jenkins/workspace/impala-asf-2.x-exhaustive-rhel7/repos/Impala/tests/custom_cluster/test_query_concurrency.py", line 58, in poll_query_page
      except e:
  NameError: global name 'e' is not defined

Change-Id: If507409b8945b16a9510bb6195343eed7d8538fc
Reviewed-on: http://gerrit.cloudera.org:8080/10049
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala