EVP: clear error when falling back from failed EVP_KEYMGMT_fetch()

EVP: clear error when falling back from failed EVP_KEYMGMT_fetch()

Since we're falling back to legacy, this isn't an error any more.
Among others the failed EVP_KEYMGMT_fetch() error shadows other errors
produced by the legacy code, which disrupts our test/evp_test runs.

We use the error stack mark to restore the error stack just right,
i.e. ERR_set_mark(), ERR_clear_last_mark() and ERR_pop_to_mark()

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/10803)