Package TWiki::Configure::Checkers::CertificateChecker 
Configure GUI checker for Certificate items.
This checker validates files that contain X.509 certificates,
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 checkUsage ($valueObject,$usage) -> $errorString 
Validates a Certificate item for the configure GUI 
-  $valueObject- configure value object
-  $usage- Required use (email, client, server, clientserver)
Returns empty string if OK, error string with any errors