Do not return PSA_ERROR_DOES_NOT_EXIST for a non-existing key

[gilles-peskine-arm]

As of version 1.0.0, PSA Crypto API functions return PSA_ERROR_INVALID_HANDLE to indicate that a key identifier that should exist does not exist, no matter the reason (potentially valid identifier that does not exist now, or permanently invalid value). Mbed TLS still returns PSA_ERROR_DOES_NOT_EXIST in some cases, including psa_destroy_key and key operations. We should return PSA_ERROR_INVALID_HANDLE instead.

PSA_ERROR_DOES_NOT_EXIST can still be returned:

• By psa_open_key.
• By psa_its_xxx and internal storage functions.
• Potentially by driver interface functions.

The goal of this task is to change code that returns PSA_ERROR_DOES_NOT_EXIST, but should not, to return PSA_ERROR_INVALID_HANDLE instead, and update tests and documentation accordingly.