Update the throttle plugin to propogate underlying errors.

[bbockelm]

Make sure to always set the error member appropriately whenever something besides SFS_OK is returned.

Fixes #752

[abh3]

Hi Brian,

Well, that's 50% of the problem. While the error information is indeed copied back, the callback information and other options are not copied forward. That means all callback are turned off in he subsequent ofs layer. Not something that is a god thing to do for performance. Please take a look at hjow this is done via XrdSsiFile::CopyECB() and when it should be called. Of course, all of this is nonsense and will be trivialized in R5.

[bbockelm]

@abh3 - ok, the latest commit copies forward the callbacks in the same manner as SSI.

[abh3]

Thanks!