SKIP instead of ERROR if opentype/slant_direction's reference codepoint is not covered

[Hoolean]

Description

opentype/slant_direction errors if a font does not cover H, as it uses its outlines to determine slant direction. There are legitimate reasons for a font to not contain an H however, and so this PR makes this case an explicit SKIP instead.

Checklist

• update CHANGELOG.md
• wait for the tests to pass
• request a review

[felipesanches]

thanks, @Hoolean