Fix Trac #14690 - :steplocal panics after break-on-error
:steplocal enables only breakpoints in the current top-level binding.
When a normal breakpoint is hit, then the module name and the break id from the BRK_FUN byte code
allow us to access the corresponding entry in a ModBreak table. From this entry we then get the SrcSpan
With this source-span we can then determine the current top-level binding, needed for the steplocal command.
However, if we break at an exception or at an error, we don't have an BRK_FUN byte-code, so we don't have any source information.
The function bindLocalsAtBreakpoint creates an UnhelpfulSpan, which doesn't allow us to determine the current top-level binding.
To avoid a panic, we have to check for UnhelpfulSpan in the function ghc/GHCi/UI.hs:stepLocalCmd.
Hence a :steplocal command after a break-on-exception or a break-on-error is not possible.
(cherry picked from commit 1be9c35c940e9a1edbb44a0e7dd51b48529ffb9b)