Improve std.get and error messages

[stephenamar-db]

This PR does 2 things:

It improves std.get() to only compute default when it's needed.

and it makes builtin error more readable
Before

bazel-bin/bazel/jsonnet/sjsonnet.jar -e 'std.get(["a", "b"], "a", null)'
sjsonnet.Error: Wrong parameter type: expected Object, got array
    at [ApplyBuiltin].(<exec>:1:8)

after

bazel-bin/bazel/jsonnet/sjsonnet.jar -e 'std.get(["a", "b"], "a", null)'
sjsonnet.Error: Wrong parameter type: expected Object, got array
    at [std.get].(<exec>:1:8)