HPCC-9073 Report better error for unsupported DISTRIBUTION types

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
True, my mistake, I forgot about the null termination being the difference between the two. Though does this not mean that varstrings should now go into the first category  HQLERR_DistributionVariableLengthX when they have no explicit size?

Ok, just tested and varstringN was still unimplemented before this fix, so that's no to my question above. I'm wondering why but I'll  query out of this channel.