Add ability to wrap an error with an Unimplemented error

I have a bit of code that does something like this:
```
errors.UnimplementedErrorf(
link,
"unsupported expression %q: %v",
exprString,
err,
)
```

This flattens the `err` argument. What I'd really like is something like

```
errors.UnimplementedWrapf(
err,
link,
"unsupported expression %q",
exprString,
)
```

(analogous to `errors.Errorf` versus `errors.Wrapf`)

The exact piece of code I'm referring to is: https://github.com/cockroachdb/cockroach/blob/7bf398fc9c715e67ae4ab342569b26f50809fb59/pkg/ccl/importccl/read_import_mysql.go#L764

cc @knz

从 pkg/ccl/importccl/read_import_mysql.go 中大约第 764 行的被引用调用开始，然后比较现有的 UnimplementedErrorf 和 Wrapf API。添加所请求的 UnimplementedWrapf 行为，使原始错误保持被包装，同时消息包含所提供的表达式文本；使用针对错误格式化的测试进行验证。

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