Update Error Message for Missing User Pin in Config File

Currently, calling `Cred` with a json config file without a user pin throws the following error: "failed to retrieve certificate chain: unexpected EOF." The error message should be made more descriptive to show that certain fields are missing from the config.

Source: https://github.com/googleapis/enterprise-certificate-proxy/blob/main/client/client.go#L159C2-L161C3

Start in client/client.go around lines 159-161 and reproduce Cred with a JSON config file that omits the user pin. Trace where the "unexpected EOF" message is produced and identify which missing configuration fields should be reported. Done means the failure describes the missing config fields instead of only reporting the certificate-chain error.

Written by the indexing model from the issue text.