mtd: tests: abort torturetest on erase errors

The torture test should quit once it actually induces an error in the
flash. This step was accidentally removed during refactoring.

Without this fix, the torturetest just continues infinitely, or until
the maximum cycle count is reached. e.g.:

   ...
   [ 7619.218171] mtd_test: error -5 while erasing EB 100
   [ 7619.297981] mtd_test: error -5 while erasing EB 100
   [ 7619.377953] mtd_test: error -5 while erasing EB 100
   [ 7619.457998] mtd_test: error -5 while erasing EB 100
   [ 7619.537990] mtd_test: error -5 while erasing EB 100
   ...

Fixes: 6cf78358 ("mtd: mtd_torturetest: use mtd_test helpers")
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: <stable@vger.kernel.org>