libctf: improve handling of type dumping errors

When dumping a type fails with an error, we want to emit a warning noting
this: a warning because it's not fatal and we can continue.  But warnings
don't automatically print out the ctf_errno (because not all cases causing
warnings set the errno at all), so we must do it at warning-emission time or
lose track of what's gone wrong.

libctf/

	* ctf-dump.c (ctf_dump_format_type): Dump the underlying error on
	type dump failure.