Convert ShellError::UnsupportedInput to named fields (#10971)

# Description

This is easy to do with rust-analyzer, but I didn't want to just pump
these all out without feedback.

Part of #10700

# User-Facing Changes

None

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/src/env/load_env.rs

@@ -81,12 +81,12 @@ impl Command for LoadEnv {
                     }
                     Ok(PipelineData::empty())
                 }
-                _ => Err(ShellError::UnsupportedInput(
-                    "'load-env' expects a single record".into(),
-                    "value originated from here".into(),
-                    span,
-                    input.span().unwrap_or(span),
-                )),
+                _ => Err(ShellError::UnsupportedInput {
+                    msg: "'load-env' expects a single record".into(),
+                    input: "value originated from here".into(),
+                    msg_span: span,
+                    input_span: input.span().unwrap_or(span),
+                }),
             },
         }
     }