ERR_error_string adds some extra information that depends on the OpenSSL
library version (code, module, method). This information says nothing to
the end user, and it results in different test results after updating to
OpenSSL 3.0. Let's use ERR_reason_error_string instead, which just
prints a human-readable error message.

Part of #6477

NO_DOC=minor change in error message
NO_CHANGELOG=minor change in error message