Remove some wrong unused warnings, do not issue warnings if there are errors.

This patch fixes to omissions when variables were not set as referenced
and removes the warnings if errors have previously been issued.

gcc/fortran/ChangeLog:

	PR fortran/30438
	PR fortran/125801
	* io.cc (gfc_resolve_dt): Set internal unit as used on read.o
	* resolve.cc (resolve_transfer): Move setting value as used
	before possible early return.
	(gfc_resolve): Only call warn_unused_vs_set if there are no
	pending errors.

gcc/testsuite/ChangeLog:

	PR fortran/30438
	PR fortran/125801
	* gfortran.dg/warn_unused_but_set_error_1.f90: New test.
	* gfortran.dg/warn_unused_intent_out_2.f90: New test.