Replace the non-X.509-named error code `MBEDTLS_ERR_OID_NOT_FOUND` with
`MBEDTLS_ERR_X509_UNKNOWN_OID`, which already exists and is currently not
used for anything.
Public functions in X.509 propagate this error code, so it needs to have a
public name.
Remove the definition of `MBEDTLS_ERR_OID_NOT_FOUND` in `x509_oid.h`, then
```
git grep -l MBEDTLS_ERR_OID_NOT_FOUND | xargs perl -i -pe 's/\bMBEDTLS_ERR_OID_NOT_FOUND\b/MBEDTLS_ERR_X509_UNKNOWN_OID/g'
```
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
4c83221320