Better error message for keyring check

[mdlavallee92]

If keyring not set then checkDatabaseCredential returns an unclear error.

Ulysses::checkDatabaseCredential(cred = "cred", db = "db", keyringName = "project")
#> ! Check that credential db_cred is correct.
#> Error in b_wincred_i_get(target): Windows credential store error in 'get': Element not found.

^{Created on 2023-07-07 with reprex v2.0.2}

The error comes from keyring....add an error message that input does not exist and suggestion to load it using Ulysses::setCredential()