Make cudnnex SDPA checkers more conservative.

[wujingyue]

#57's description and https://github.com/Lightning-AI/lit-thunder-LEGACY/pull/2480#issuecomment-2013537240 have the context.

The proposed improvement is to properly propagate graph-not-supported errors from the cudnn backend to the frontend as distinguishable exceptions. This way, we can make the checkers to only reject unsupported graphs and not hiding other programming errors.