set_margins: raise a clear error for a negative margin - #2094

set_margins(), set_left_margin(), set_top_margin() and set_right_margin() accepted a negative margin, so text was written past the edge of the page. They now raise ValueError: Left margin must be zero or a positive number (or Top / Right), like set_auto_page_break() does for the bottom margin since #2084.

set_margins() checks all its margins before changing any, so an invalid one leaves the others unchanged. Its default right=-1 (same as the left margin) still works.

• 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