Print explicit error message on iSCSI boot failure, and add

"Permission denied" as an error text to strerror().

src/hci/strerror.c

 struct errortab einval __errortab = { EINVAL, "Invalid argument" };
 struct errortab enospc __errortab = { ENOSPC, "No space left on device" };
 struct errortab eio __errortab = { EIO, "Input/output error" };
+struct errortab eacces __errortab = { EACCES, "Permission denied" };

src/tests/iscsiboot.c

 #include <stdint.h>
+#include <string.h>
 #include <byteswap.h>
 #include <vsprintf.h>
 #include <gpxe/netdevice.h>
 
 	printf ( "Initialising %s\n", target_iqn );
 	if ( ( rc = init_iscsidev ( &test_iscsidev ) ) != 0 ) {
-		printf ( "Could not reach %s\n", target_iqn );
+		printf ( "Could not reach %s: %s\n", target_iqn,
+			 strerror ( errno ) );
 		return rc;
 	}
 	ibft_fill_data ( netdev, initiator_iqn, target, target_iqn );