`vaultenv` secrets file parsing issues

If one creates a secrets file like this:
```
VERSION 2

mount secret
example#secret
```

and run `vaultenv` with this secret file, it will not load any of the secrets defined beyond the `mount` statement.
This is unexpected behavior, I would expect a parse error (preferably with a suggestion on how to fix the file).
If `VERSION` is written with lowercase letters, we do get a parse error:
```
[ERROR] example.secrets:1:8:
|
1 | version 2
| ^
unexpected space
expecting '#' or path component
```

If any other lowercase string is put on a single line (without `#` or `/`), we get the same behavior: any secrets below it are secretly ignored.

Start by reproducing the issue with the supplied secrets file and the vaultenv command, then trace the secret-file parser and its existing error handling. Done means a standalone invalid line such as `VERSION 2` or another lowercase string no longer silently stops parsing: vaultenv should report a parse error with a useful correction suggestion.

Written by the indexing model from the issue text.