CLN:Better exceptions in common, io/excel & format

* more changes to bare exceptions
* fixup _mut_exclusive to be more expressive
* change interpolate_2d and _maybe_upcast_putmask to assertion errors
  (with pragmas) Potentially need to check whether these are actually
  unreachable in normal code...
* CLN: Add coverage pragmas to already guarded asserts in CSVFormatter
* CLN: io/excel Exceptions - small changes (mostly pragmas and asserts)