fastboot: bail out if failed to generate fs image

we don't want to continue fastboot process if failed to
generate fs image. Print an error message and exit early.

Bug: 64915319
Change-Id: I5506d2a7a5063c188685633d6c3890239f9d658e

fastboot/fastboot.cpp

@@ -1463,7 +1463,7 @@ static void fb_perform_format(Transport* transport,
 
     if (fs_generator_generate(gen, output.path, size, initial_dir,
             eraseBlkSize, logicalBlkSize)) {
-        fprintf(stderr, "Cannot generate image: %s\n", strerror(errno));
+        die("Cannot generate image for %s\n", partition);
         return;
     }