show descriptions for shader label and mesh attribute errors in material editor

[ScanMountGoat]

This should show if the shader label is invalid and what required attributes are missing. Placing a red/yellow checkerboard icon next to the errors could make the errors more intuitive. This also requires knowing what meshes are assigned to the given material and what attributes are missing for each mesh.

• [x] show if shader label is invalid
• [x] show missing required attributes