Report dlsym, dlvsym lookup errors using dlerror [BZ #19509]

	* elf/dl-lookup.c (_dl_lookup_symbol_x): Report error even if
	skip_map != NULL.
	* elf/tst-dlsym-error.c: New file.
	* elf/Makefile (tests): Add tst-dlsym-error.
	(tst-dlsym-error): Link against libdl.

(cherry picked from commit 7d45c163)