fix: log append reports an unreadable --body-file as ERROR, not a traceback (#145)

Files changed:
- CHANGES.md
- VERSION
- tools/CONTRACT.md
- tools/chemenu/commands/log_append.py
- tools/chemenu/tests/test_log_append.py

tools/CONTRACT.md

@@ -999,12 +999,12 @@ Append a formatted entry to `kb/log.md`.
 
 - 0    success
 - 1    `--op` is not one of ingest, query, lint, create, update, delete, rename, move
-- 1    `--body-file` cannot be read
+- 1    `--body-file` is missing, not a readable file, or not valid UTF-8
 
 **ON FAILURE**
 
 - `--op` is not one of ingest, query, lint, create, update, delete, rename, move -> Nothing was written - fix the argument and retry once
-- `--body-file` cannot be read -> Nothing was written - fix the path and retry once
+- `--body-file` is missing, not a readable file, or not valid UTF-8 -> Nothing was written - fix the path and retry once
 
 **NEVER**