drm: fix error atom plumbing

* new error enums
* remove session from map *after* close
* mark security level optional for error atom
* initialize enums to 0

TODO(b/275729711): reclaim sessions that failed to close

Bug: 266444896
Test: MediaDrmAtomTests
Change-Id: I0b1155e9db16f5e9fd44d8b2ff92cfbe0adf23c1