hg 5.9 censor: adapted error triggering in RSpec tests

It seems that tombstone size is not a problem anymore. On the other
hand, censoring a head is still an error.

--HG--
branch : heptapod
12 jobs for !293 with topic/heptapod/hg-5.9 in 29 minutes and 25 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed rubocop #254908

00:09:59

 
  Test
passed rspec-regular 1/8 #254912

00:16:27

passed rspec-regular 2/8 #254913

00:16:53

passed rspec-regular 3/8 #254914

00:16:08

passed rspec-regular 4/8 #254915

00:15:54

passed rspec-regular 5/8 #254916

00:15:55

passed rspec-regular 6/8 #254917

00:19:11

passed rspec-regular 7/8 #254918

00:18:32

passed rspec-regular 8/8 #254919

00:13:30

passed workhorse:test using go 1.15 #254910

00:08:02

passed workhorse:test using go 1.16 #254911

00:08:23

passed workhorse:verify #254909

00:07:06