Hard-code op name in ck_exists|delete errors

Hard-code op name in ck_exists|delete errors

The reason is manifold:
• ck_exists and ck_delete are each only ever called with one op type,
  so we can hard-code the op, which takes less code.
• The messages in perldiag are more helpful if they include
  the op name.
• Changing them in op.c too avoids the need for
  diag_listed_as clutter.

(I know, two of these are not listed in perldiag yet.  I’m get-
ting to that.)