Bad table name on master_create_distributed_table()

This fix handles when master_create_distributed_table is called with
a an index as the first argument. More generally, it is called with
a relation that is not ordinary table or foreign table.

closes #13 
Maybe change wording to "cannot" instead of "could not"? See postgres error message style guidelines regarding past vs present tense. Since this can never succeed, we could frame it as "cannot".