improve and test iApply error message

4 jobs for gen_proofmode in 16 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #14267
fp
build-coq.8.7.1

00:04:58

passed #14266
fp
build-coq.8.7.2

00:05:13

passed #14265
fp-timing
build-coq.8.8.0

00:04:39

passed #14264
fp
build-coq.dev

00:16:21