abi{dw,diff}: Better error messages when alternate debuginfo not found

abi{dw,diff}: Better error messages when alternate debuginfo not found

When the alternate debug info referenced by the main debug info file
is missing, both abidw and abidiff fail to explain what is happening
to the user.  The patch adds explicit error messages to those two
programs in that case.

	* tools/abidiff.cc (handle_error): Handle cases where the
	fe_iface::STATUS[_ALT]_DEBUG_INFO_NOT_FOUND bits are set. Refer to
	the alternate debug info file in the error message.
	* tools/abidw.cc (load_corpus_and_write_abixml): Do not clear the
	reader before emitting the error message, rather clear it after.
	Also, refer to the alternate debug info file in the error message.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>