+ * return 0 for anything (including invalid certificates)
+ * other than fatal error, as a non-zero return code
+ * immediately aborts the verification process. For fatal
+ * errors, a specific error code should be used (different
+ * from MBEDTLS_ERR_X509_CERT_VERIFY_FAILED which should not
+ * be returned at this point), or MBEDTLS_ERR_X509_FATAL_ERROR
+ * can be used if no better code is available.