Reverted the change with the error handling additions based on decisions of WG to pull that language.

My review indicates that there are mainly two areas of concern:

• GCM

• The subject-id error handling bits

The latter we're still debating but I do have it implemented, it's just what final form we want to include it in.

The GCM thing is certainly not the default but if the EncryptionMethod elements for it are included in metadata it should work fine, need to test that. For sources like InCommon where that's not possible to control, it seems like it's worth supplying a predefined SecurityConfiguration with that turned on, and in theory that could be metadata tag-driven as a workaround.