Display value when raising due to unscope() issues

Hopefully make it easier to debug errors. e.g

Before:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String is unimplemented.

After:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String "'t'='t'" is unimplemented.
Merge pull request #13848 from huoxito/tell-value

Display value when raising due to unscope() issues