FAPI: Improve the error message for self signed EK certificates.

[JuergenReppSIT]

The error message "curl_url_set for CURUPART_URL failed: Unsupported URL scheme" was displayed if a self signed EK certificate was stored in the TPM. Now a better error message is displayed to explain that FAPI can be used if "ek_cert_less" is set to "yes" in the FAPI config file. Addresses: #2833