bake: format hcl errors with source definition

[tonistiigi]

depends on #389

Extend typed errors support to cover bake errors from the client side:

/work/hello # docker buildx bake
docker-bake.hcl:3
--------------------
   1 |     target "binary" {
   2 |       target = "bin"
   3 | >>>   plat forms = ["local"]
   4 |       output = ["type=local,dest=bin/"]
   5 |     }
--------------------
error: docker-bake.hcl:3,14-15: Invalid block definition; The equals sign "=" indicates an argument definition, and must not be used when defining a block.

/work/hello # docker buildx bake --print -f docker-bake.json foo
docker-bake.json:3
--------------------
   1 |     {
   2 |     	"target": {
   3 | >>> 		"foo": {},
   4 |     	}
   5 |     }
--------------------
error: docker-bake.json:3,14-15: Trailing comma in object; JSON does not permit a trailing comma after the final property in an object.