[BUG] - `renderFileError` shows the file path twice for `FileIOError`

Reported by @jasagredo.

When a file cannot be read, the rendered `FileIOError` message contains the path twice:

```
blah.json: blah.json: withBinaryFile: does not exist (No such file or directory)
```

Expected, with the path once:

```
blah.json: withBinaryFile: does not exist (No such file or directory)
```

To reproduce, render the error from any reader in `Cardano.Keys.IO` (added in #4) on a file that does not exist:

```haskell
Left err <- readFileTextEnvelope "blah.json"
:: IO (Either (FileError TextEnvelopeError) (SigningKey KesKey))
Text.putStrLn $ docToText $ renderFileError renderTextEnvelopeError err
```