identity: fixes to the eos-after and error-after properties

I copied `error-after` to make the `eos-after` property, but it turned
out there were some problems with that one, so this patch: adds
separate counters (so setting to NULL and reusing the element will
still work); clarifies the properties' min values; and reports an
error when both are set.
6 jobs for more-identity-after-stuff in 27 minutes and 23 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #67251
gst indent

00:00:43

passed #67250
manifest

00:00:25

 
  Build
passed #67252
build fedora x86_64

00:03:00

 
  Test
passed #67253
check fedora

00:15:45

 
  Full Builds
passed #67254
build android arm64 api28

00:03:45

passed #67255
build cerbero fedora x86_64

00:07:53