Improve the error message for STRUCT to STRUCT casts

[taniabogatsch]

See #10486.

We should also update this in our documentation. I think rewriting the code to support rearranging the child vectors is not necessary?

CREATE TABLE t1 (s STRUCT(str STRING, i INT));
CREATE TABLE t2 (s STRUCT(i INT, str STRING));
INSERT INTO t1 SELECT {'str': 'hello', 'i': 42};
INSERT INTO t2 SELECT {'i': t1.s.i, 'str': t1.s.str} FROM t1;
FROM t1;
┌────────────────────────────────┐
│               s                │
│ struct(str varchar, i integer) │
├────────────────────────────────┤
│ {'str': hello, 'i': 42}        │
└────────────────────────────────┘
FROM t2;
┌────────────────────────────────┐
│               s                │
│ struct(i integer, str varchar) │
├────────────────────────────────┤
│ {'i': 42, 'str': hello}        │
└────────────────────────────────┘

[plaflamme]

FWIW: this came up while doing something like

df = pd.DataFrame([{"s":{"a": "foo", "b": 42}}]) # some dataframe with a struct
con.query("INSERT INTO t1 SELECT * FROM df")

(In my particular case, df came from BigQuery, e.g.: df = job.to_dataframe())

In this case, it's not always possible to control the order of the struct members. When the struct is backed by pyarrow, then I believe this is possible, but when it's a python object, I'm unsure it is possible to have a strict / consistent / predictable ordering.

[Mytherin]

Superseded by #10537