Convert Shellerror::GenericError to named fields (#11230)

# Description

Replace `.to_string()` used in `GenericError` with `.into()` as
`.into()` seems more popular

Replace `Vec::new()` used in `GenericError` with `vec![]` as `vec![]`
seems more popular

(There are so, so many)

crates/nu-command/src/system/run_external.rs

@@ -601,16 +601,16 @@ impl ExternalCommand {
             }
             process
         } else {
-            return Err(ShellError::GenericError(
-                "Current directory not found".to_string(),
-                "did not find PWD environment variable".to_string(),
-                Some(span),
-                Some(concat!(
+            return Err(ShellError::GenericError{
+                error: "Current directory not found".into(),
+                msg: "did not find PWD environment variable".into(),
+                span: Some(span),
+                help: Some(concat!(
                     "The environment variable 'PWD' was not found. ",
                     "It is required to define the current directory when running an external command."
-                ).to_string()),
-                Vec::new(),
-            ));
+                ).into()),
+                inner:Vec::new(),
+            });
         };
 
         process.envs(&self.env_vars);