A refused or failed write is an err record in the log, naming the file and the reason

Through a mount every refusal reads as Invalid argument, so a shell user
could not tell a missing regexp match from a bad ctl word. Every write or
truncation the tree refuses or that fails now adds err <serial|-> <file>:
<why> to /log. No per-pane readable error file: acme's errors only takes
text, and one stream is simpler to watch.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 7 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 714a8d2c..6eb56537 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -297,7 +297,13 @@ an open renders its frame; stat the entry.
 `new`, `del`, `rename` (a terminal's too, as its shell changes directory,
 since a terminal is named by its directory) and `save <serial> <name>`, and `msg <serial|-> <text>`
 for every line the editor says, repeats included (with `verbose` on, that
-includes each builtin announcing itself as it runs). A `msg` said while a
+includes each builtin announcing itself as it runs), and `err <serial|->
+<file>: <why>` for every write or truncation the tree refused or that
+failed -- through a mount a shell sees only the errno its kernel mapped the
+reply to, usually `Invalid argument`, and this is the reason (`err 3 addr:
+no match for regexp`). There is no per-pane error file to read instead:
+acme's `errors` only takes text, and one record stream is simpler to watch
+than a file per pane. A `msg` said while a
 pane is being made can precede that pane's `new`; panes present at boot are
 recorded before anything else.
 Control characters in a record become spaces, so a record is one line. An