Fixes#2123
Slight breaking change if someone had an expectation that no copies
were made behind the scene (which is sensible), however writing code
that relies on this expectation is probably buggy anyway.
Previously, calls to add_line in add_command_formatting of default help
commands would fail if the command's help message would overflow the
current page. This would also result in silent failure as the
RuntimeError raised from add_line is never caught.
This patch adds behavior that adds lines individually should it raise,
which guarantees safe pagination as long as every line is smaller than
the maximum page size, which is highly unlikely.
