transcode: disable pcr forwarding on error
This merge request introduce a new runtime error in the pcr_helper to replace an assert.
This assertion was too severe and deserve actual runtime handling.
The pcr_helper should just fail in case of inconsistent
input/output. This can happen in various case such as bogus encoder or
simply internal error in the pcr_helper code.
This patch is making sure the pcr_helper user can disable it in case
of failure.