dhall --ascii --plain does not work for errors

$ dhall --plain --ascii <<< ./foo.dhall

```
Use "dhall --explain" for detailed errors

↳ ./foo.dhall

Error: Expression doesn't match annotation

- Text
+ Natural

4 : Text
```

We’ve got unicode, and colors (the “Error” is still displayed in red).

Same with parser errros:

```
$ dhall --plain --ascii <<< ./generators.dhall

↳ ./generators.dhall

Error: Invalid input

/home/philip/kot/dhall/generators/generators.dhall:2:1:
|
2 |
| ^
unexpected end of input
expecting expression or whitespace
```

(the `Errors` contains colors)

Start by running the two reported `dhall --plain --ascii` commands against the example inputs and inspect the CLI error-rendering entry point. Done means both type and parser errors contain no Unicode characters and no color escape sequences when those flags are used.

Written by the indexing model from the issue text.