sample: tfm: Improve the error handling at tfm_secure_partition NS-side

- Previously non-secure request at the 6th run expected to return status
  "-135 (PSA_ERROR_INVALID_ARGUMENT)" which is the expected status so
  updating "if" condition on the non-secure side handles this expected
  type error status from the secure side.
- update sample YAML harness config regex of Digest message as this gets
  compared at the run of twister.
- Update the readme console logs.

Signed-off-by: Rajkumar Kanagaraj <rajkumar.kanagaraj@linaro.org>