Ignore exit(1) from Puppet 3.4 parser face

The parser face now exits with a non-zero exit code if it encounters any
errors in the manifests it is testing:

puppetlabs/puppet@1a526d0

This is useful for the `puppet parser validate` CLI. But it's not useful to
us here, so catch and ignore the `SystemExit` exception.

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This supersedes #4 (re-implemented per comments) and #5 (included in this PR).