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