Fix #22742

runtimeRepLevity_maybe was panicing unnecessarily; and
the error printing code made use of the case when it should
return Nothing rather than panicing.

For some bizarre reason perf/compiler/T21839r shows a 10% bump in runtime
peak-megagbytes-used, on a single architecture (alpine). See !9753 for
commentary, but I'm going to accept it.

Metric Increase:
    T21839r