DAP: Set reason to failed for unverified breakpoints

[jsternberg]

Description

The DAP specification allows us to specify a reason why a breakpoint is unverified. Every breakpoint starts as unverified because we don't have enough information to know whether a breakpoint is valid on launch, but we do learn which ones are valid and mark them as verified. We don't update the reason for why a breakpoint is unverified from pending to failed when it doesn't correspond to a real instruction.