When a choice has been discriminated, the diagnostics if the discriminated branch fails should NOT include
Because a discriminator isolates things to one branch.
A choice could backtrack through a few alternatives, eventually hit a discriminator, and then fail. There may be branches that were never attempted (after the branch where the discriminator occurred) hence, it is simply wrong to say all alternatives failed, as there were potentially alternatives that were never tried.
But really, once a discriminator has been evaluated to true, then the choice should not be creating any diagnostics about the choice itself, or at worst it should be saying "Choice Discriminated Branch Failure" only about the one branch.