fix: discard-dist-on-failure keeps the step's status when its message cannot be written (closes #342)

With stderr closed, the wrapper's message write failed and set -e ended it
with status 2; with stderr a pipe whose reader had gone, the write killed it
with 141. Either replaced the failed step's own status. The message is now
written with SIGPIPE ignored and its failure ignored, after the step has run,
so the step still sees the default SIGPIPE. The header now states that an
interrupt removes nothing (it is not a build failure) and that a failed
check-censored --require-dist removes dist/ (a dist/ not cleared of the barred
name must not ship); neither behaviour changed. Also the README bullet's
missing period.

Model: opus-5-5

README.md

@@ -277,7 +277,7 @@ provide:
   step's exit status, saying on stderr that it did and why. Every step of
   `make build` runs through it; `make build-debug` runs none of them through it.
   A step that succeeds removes nothing, and a removal that cannot be completed
-  is reported as loudly as one that was
+  is reported as loudly as one that was.
 - `script/test-verify-build` — exercise every failure mode of
   `script/verify-build` against a fixture tree in a temp dir, asserting the exit
   status and the message of each, assert the state of `dist/` on disk after a