Improve error messages about address space

[kvark]

It's a bit annoying to see this kind of an error:

error: 
  ┌─ uniform.wgsl:2:4
  │
2 │ var buffer: array<vec4<f32>, 4>;
  │    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ naga::GlobalVariable [1]

Global variable [1] 'buffer' is invalid: 
        Type isn't compatible with with the storage class

It's unclear what storage class is meant, and what the problem is. This PR changes this into:

error: 
  ┌─ uniform.wgsl:2:4
  │
2 │ var buffer: array<vec4<f32>, 4>;
  │    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ naga::GlobalVariable [1]

Global variable [1] 'buffer' is invalid: 
        Type isn't compatible with with address space Handle

Which is a bit better...

@jimblandy I feel that defaulting to Handle class is not doing WGSL any good, and it should be explicit instead.