fix: correctly print multi-line errors

[tigarmo]

The bug manifested in a situation like this:

• A long progress() message is printed, say "AAAAAAAA";
• An error is emitter, and the result of str(error) is a multi-line string where the first line is shorter than the previously-emitted progress() message. Example: "BBBB\nCCCC";
• The newline in the error message is not correctly handled by the printer, meaning that after "BBBB" is written to the terminal the remainder of the line is not cleared. So the end result looks like:

  BBBBAAAA CCCC

The "proper" fix for this would mean a big rework on the printer to properly support multi-line messages; for now, fix only the error case by "manually" splitting the error message and printing each line individually.

Fixes #263

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[lengau]

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