Way to tell error messages with different compliance levels.

Discussed in https://github.com/opencontainers/runtime-tools/pull/354. @wking recommends we can using [TAG native diagnostics](https://testanything.org/tap-version-13-specification.html#diagnostics) to tell the different compliance levels of validating messages.  
