Add hint to `influx write` if CLI arg isn't valid LP

`influx write` supports reading data as a positional arg, or from a file. It's easy for users to get mixed up and accidentally pass a filename as a positional arg, leading to error messages like:
```
Error: failed to write data: 400 Bad Request: unable to parse 'latest-observations.lp': missing fields
```
It'd be nice to add some sort of hint about missing the `--file` flag to the error message, if we can figure out a heuristic for detecting this type of error.

Start at the `influx write` command and trace how positional arguments and file input are parsed and how the shown write error is surfaced. Define and test a reliable heuristic for likely filenames passed as data, then confirm the hint appears only for that case and that normal invalid line-protocol errors remain unchanged.

Written by the indexing model from the issue text.