mtd: tests: incorporate error message for mtdtest_read()

All callers of mtdtest_read() print the same error message on failure.
This incorporates the error message to mtdtest_read() and removes them
from the callers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>