[m-rev.] diff: rewrite error_pieces_to_string

[Zoltan Somogyi]

On 2023-08-05 18:46 +02:00 CEST, "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> This should not need review.

Apparently, it could have used some :-(

That diff caused test failures by inserting newlines into error messages.
I bootchecked it, but added the newlines afterward, because they are essential
to making the output of error_msg_inst actually readable.

The attached diff fixes the test failures while preserving this intended use.

Zoltan.
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