libctf: improve ECTF_NOPARENT error message

This erorr doesn't just indicate that there is no parent dictionary
(that's routine, and true of all dicts that are parents themselves)
but that a parent is *needed* but wasn't found.

include/
	* ctf-api.h (_CTF_ERRORS) [ECTF_NOPARENT]: Improve error message.

ld/
	* testsuite/ld-ctf/diag-parname.d: Adjust.