Fix AttributeError when test_function_help fails

This commit fixes a missed usage of the CommandFailed exception during
the migration moving it from tempest.cli to tempest.exceptions in change
Ibf5f1cbbb847d32976b54c4484acfc3c0e3b4f48. The outdated module
path could cause an AttributeError to be raised because the exception no
longer existing in tempest.cli.

Change-Id: I502eaef3486d903931dab43069cec1ae7ee570f1
Merge "Fix AttributeError when test_function_help fails"