rxgen: Error out if cpp exits with error

rxgen: Error out if cpp exits with error

If we call 'cpp' and it returns with an erroneous exit code, exit with
an error ourselves. Otherwise we will return success and generate
empty results even if the input file is unreadable, for instance,
which causes confusing errors later in the build.

Change-Id: Ia12dc58eee0937a6bd529dcd6b552e253dbc40c8
Reviewed-on: http://gerrit.openafs.org/2519
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>