### `wai`, `wai-pack` and `wai-quantum` end quietly when their reader stops reading

- **A reader gone** (`wai channel verify ... | head`) ends `wai` without a word, as it ends a Unix
  tool: on Unix by `SIGPIPE`, which a shell reports as 141, and on Windows with status 141.
  Neither is one of the toolchain's exit codes. Until now the next write panicked ("failed
  printing to stdout: Broken pipe") and `wai` exited 7, an internal error. Any other failure to
  write standard output is still the error it was. Only a write to the process's own standard
  output or error ends it so: a write to a child's input that has gone is still reported by the
  command that made it.
- **`wai-pack` and `wai-quantum`** do the same. `wai-quantum completions` writes its script whole,
  so a failed write is its error (exit 1) rather than a panic.
- **`wai logs` and `wai session list`**, following a log, ended with success on any failed write;
  one other than a reader gone is now an internal error (exit 7).