Rollup merge of #60897 - seanmonstar:patch-4, r=sfackler

error: remove StringError from Debug output

Seeing `StringError("something something")` in debug output can cause
 someone to think there was an error dealing with `String`s, not that the
error type is just a string. So, remove that noise.

For example:

```
io error: Custom { kind: InvalidData, error: StringError("corrupt data") }
```

With this change:

```
io error: Custom { kind: InvalidData, error: "corrupt data" }
```

diff --cc src/libstd/error.rs
Simple merge