missing shader def literal should report appropriate error

[robtfm]

currently trying to use a shader def literal when the def isn't defined gives a strange error:

error: expected expression, found '#'
  ┌─ crates/bevy_pbr/src/render/pbr_bindings.wgsl:6:101
  │
6 │ var<uniform> materials: array<bevy_pbr::pbr_types::StandardMaterial, #{MATERIAL_BUFFER_BATCH_SIZE}u>;
  │                                                                                                     ^ expected expression
  │
  = expected expression, found '#'

it should instead return a specific error pointing out the missing def.