improve serde error messages

[wez]

In this scenario:

struct Foo {
  foo: bool
}

deserializing this:

foo: "bar"

would yield an error with no context. This commit includes the
field name in the error message for this and a couple of other
similar looking cases.

It would be great if the filename and line number of the originating
token were available for these error messages; the use case I'm
considering would benefit from using the include feature of hocon
and I can see it being frustrating to track down errors without that.

[mockersf]

👍

[mockersf]

It would be great if the filename and line number of the originating
token were available for these error messages

I'll give a go at filename, I think I have an idea on how to do it. line number would be... harder