Clear error stack on successful OSSL_STORE_open()

Since OSSL_STORE_open() tries with the 'file' scheme loader first, and
then on the loader implied by the URI if the former fails, the former
leaves an error on the error stack.  This is confusing, so let's clear
the error stack on success.  The implementation uses ERR_set_mark,
ERR_pop_to_mark and ERR_clear_last_mark to make sure caller errors are
preserved as much as possible.

Fixes #4089

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4094)