Auto merge of #55933 - euclio:doc-panic, r=QuietMisdreavus

emit error when doc generation fails

Fixes #41813.

The diagnostic looks something like this:

```
error: couldn't generate documentation: No space left on device (os error 28)
  |
  = note: failed to create or modify "/path/to/crate/target/doc/src/lazycell"
```

src/librustdoc/html/render.rs

@@ -202,8 +202,8 @@ impl Impl {
 
 #[derive(Debug)]
 pub struct Error {
-    file: PathBuf,
-    error: io::Error,
+    pub file: PathBuf,
+    pub error: io::Error,
 }
 
 impl error::Error for Error {