Fix #14690 - :steplocal panics after break-on-error. Backport to 8.8.2

: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 (see compiler/main/InteractiveEval.hs:bindLocalsAtBreakpoint). 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.