Use face text property for error buffer buttons

[cichli]

Fixes #1033.

[vspinu]

Confirmed. That fixes it. But it's strange as font-lock-face is designed to be used in font-lock modes instead of face.

[vspinu]

Are there other usages of font-lock-face also broken or it only concerns buttons?

[cichli]

Yeah, I haven't dug in too deeply but this seems to be a button.el specific thing. As far as I can tell this is the only place where we're not using the default button face on buttons (we also use buttons in apropos, doc, and test modes, but I'm not seeing this specific issue in any of them).

[bbatsov]

👍 Guess I overdid it when trying to fix the REPL font-locking. :-)