Improve RustDocFingerprint::remove_doc_dirs

We were not filtering the cases where the doc folder
paths did not exist.
Also, we were overrwriting the error when it doesn't
make sense. We now return the first one by folding the
results of the doc_dirs removal resolutions.

src/cargo/core/compiler/build_context/target_info.rs

@@ -774,16 +774,13 @@ impl RustDocFingerprint {
     }
 
     fn remove_doc_dirs(doc_dirs: &Vec<&Path>) -> CargoResult<()> {
-        let errs: Vec<CargoResult<()>> = doc_dirs
+        doc_dirs
             .iter()
+            .filter(|path| path.exists())
             .map(|path| paths::remove_dir_all(&path))
             .filter(|res| res.is_err())
-            .collect();
-        if !errs.is_empty() {
-            Err(anyhow::anyhow!("Dir removal error"))
-        } else {
-            Ok(())
-        }
+            .collect::<CargoResult<Vec<()>>>()
+            .map(|_| ())
     }
 
     /// This function checks whether the latest version of `Rustc` used to compile this