RelationKey: better validation
RelationKey: better validation

See @7andrew7's comments:

https://github.com/uwescience/myria/commit/461942e2adbc82cc8251e1056b3bfd9dcc735c5e#commitcomment-4861870

Signed-off-by: Daniel Halperin <1a02732a15f637c31107256cbee5819e36aa7c67@cs.washington.edu>

bsalimi/myria,bsalimi/myria,uwescience/myria,jamesmarva/myria,jamesmarva/myria,jamesmarva/myria,uwescience/myria,bsalimi/myria,uwescience/myria