Erroneous underlining in error buffer

[cichli]

• The filter buttons are always underlined, even when toggled off
• All frames are underlined

e.g. with font-lock-mode enabled/disabled:

It seems the default button face is always being used as button only cares about the face property and not the font-lock-face property. If I change cider-stacktrace.el to use face when rendering the buttons and frames, all is better with both font-lock-mode enabled and disabled:

Would you accept a PR to this effect? The change to font-lock-face was in 401e819, to fix #749 - but I think in this particular case, things like parenface and rainbow-delimiters don't really apply. Thoughts?