pm-process.el: hide the stderr buffer
Renamed the per-verb stderr buffer from `*pm: <verb>*` to
` *pm-err: <verb>*` so it stays out of the buffer list. Emacs's
implicit stderr-pipe sentinel writes a "Process … stderr finished"
line on every successful exit; surfacing that buffer to the user
created startup noise (e.g. `pm-refresh` on `after-init-hook`).
`pm--surface-error` still reads the same buffer for `display-warning`
text on non-zero exits — only the visibility changed.