Differentiate between malformed pickle objects and trailing junk data.

Adds the OLM_PICKLE_EXTRA_DATA error code. We fail with this code when
the pickle object looks right except for some unexpected trailing bytes
which we didn't process.
5 jobs for dkasak/fix-unpickling-error-handling in 2 minutes and 21 seconds (queued for 20 seconds)
Status Name Job ID Coverage
  Build
passed build:lib #26995
docker

00:01:23

 
  Test
passed test:lib #26996
docker

00:00:57

 
  Trigger
passed trigger:android

passed trigger:javascript

passed trigger:python