[buildcff2vf] clarify reason for VarLibCFFPointTypeMergeErrors

[josh-hadley]

Description

Improve reporting of underlying problem when VarLibCFFPointTypeMergeError is encountered in buildcff2vf

Closes #1312

Checklist:

• [x] I have followed the Contribution Guidelines
• [x] I have added test code and data to prove that my code functions correctly
• [x] I have verified that new and existing tests pass locally with my changes
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] ~I have made corresponding changes to the documentation~

[josh-hadley]

Once https://github.com/fonttools/fonttools/pull/2216 gets merged & packaged in fontTools, I'll adjust and re-open this PR.

[josh-hadley]

No action on this, closing.