CLEANUP: ssl: make ssl_sock_load_cert*() return real error codes

CLEANUP: ssl: make ssl_sock_load_cert*() return real error codes

These functions were returning only 0 or 1 to mention success or error,
and made it impossible to return a warning. Let's make them return error
codes from ERR_* and map all errors to ERR_ALERT|ERR_FATAL for now since
this is the only code that was set on non-zero return value.

In addition some missing comments were added or adjusted around the
functions' return values.