Layout constraints index out of bounds should give errors in editor instead of during built

[MeAmAnUsername]

Given

Exp.Add = [[l:Exp] + [r:Exp]]  {left, layout(offside 3)}

(I didn't know what the 3 meant as the documentation does not explain it well)

Currently: only gives error when building with Command+B.

Errors occurred during the build.
Errors running builder 'Spoofax LWB language definition builder' on project 'rpgdsl'.
index out of bounds: index is 3 but only 3 children available (index is 0-based)

Proposal: give this error already in editor, with an error marker on the 3.

Same happens for layout(indent 0 3), I assume it happens with other layout constraints as well.