set_auto_page_break: raise a clear error for a negative margin - #2084

set_auto_page_break(True, margin=-20) placed the page break 20 mm below the bottom edge of the page, so the last lines before each break were written off the page and lost. It now raises ValueError: Bottom margin must be zero or a positive number, like set_line_width() does for a negative width.

set_margin() passes its value to set_auto_page_break() too. It now does that first, so a negative margin raises before any margin is changed.

• A unit test is covering the code added / modified by this PR
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged