Configure GUI checker for Certificate Key items.
This checker validates files that contain private key files
such as for the S/MIME signatures and for the Tasks framework.
It must be subclassed for the various certificate types, as the requirements
are slightly different.
ObjectMethod check ($valueObject,$password) -> $errorString
Validates a Key item for the configure GUI
$valueObject - configure value object
$password - Optional password key name for this file
A lot of checking is done here to prevent mystery errors at runtime.
Returns empty string if OK, error string with any errors