Use a plain exception rather than CalledProcessError

Because of http://bugs.python.org/issue9400, unpickling of exceptions with
required arguments doesn't work. This includes CalledProcessError and
means nonzero exit codes are masked by an opaque error message about
arguments to __init__ and the test doesn't abort as it should.
Merge pull request #2944 from aidanhs/aphs-remove-calledprocesserror

Use a plain exception rather than CalledProcessError