Fail earlier if stale Flymake report functions called

If a Flymake backend calls a "stale" report function,
flymake--handle-report might be called for a backend function that is
no longer in the flymake--backend-state hash table.  This patch makes
that erroneous situation slightly more explicit.

* lisp/progmodes/flymake.el (flymake--handle-report): Improve
error reporting.
3 jobs for master in 9 minutes and 24 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Fast
passed #16679
test-fast-inotify

00:07:00

 
  Normal
passed #16680
test-lisp-inotify

00:02:24

passed #16681
test-lisp-net-inotify

00:01:52