Limit -ENOEXEC to mean "this is not in my format".

src/image/elf.c

 	/* Check segment lies within image */
 	if ( ( phdr->p_offset + phdr->p_filesz ) > image->len ) {
 		DBG ( "ELF segment outside ELF file\n" );
-		return -ENOEXEC;
+		return -ERANGE;
 	}
 
 	/* Find start address: use physical address for preference,
 		dest = phdr->p_vaddr;
 	if ( ! dest ) {
 		DBG ( "ELF segment loads to physical address 0\n" );
-		return -ENOEXEC;
+		return -ERANGE;
 	}
 	buffer = phys_to_user ( dest );
 
 		if ( phoff > image->len ) {
 			DBG ( "ELF program header %d outside ELF image\n",
 			      phnum );
-			return -ENOEXEC;
+			return -ERANGE;
 		}
 		copy_from_user ( &phdr, image->data, phoff, sizeof ( phdr ) );
 		if ( ( rc = elf_load_segment ( image, &phdr ) ) != 0 )

src/include/gpxe/image.h

 	 * discarded after this call; the method must preserve any
 	 * information it may require later (e.g. the execution
 	 * address) within the @c image structure.
+	 *
+	 * The method should return -ENOEXEC if and only if the image
+	 * is not in the correct format.  Other errors will be
+	 * interpreted as "I claim this image format, but there's
+	 * something wrong with it that makes it unloadable".  In
+	 * particular, returning -ENOEXEC will cause the image probing
+	 * code to try the next available image type, while returning
+	 * any other error will terminate image probing.
 	 */
 	int ( * load ) ( struct image *image );
 };