Warning for version byte mismatch should be reported for MXF asset (not for CPL)

[fschleich]

Version byte mismatches are ignored as of #412

CPL validation relies on regXML to extract MXF descriptors, and regXML does report a Warning in case of unexpected version bytes. However, that Warning is reported for the CPL, rather than the MXF asset directly.

The MXF parsing/validation code would require a refactoring to report anything but fatal errors (via exception). Should be revisited as part of a larger MXF cleanup/refactoring.