Improve serde error output (#982)

In our tests using the serde impl on `Url`, we found that failing URL serialization says the inverse of what it should say.

```toml
[project]
name = "foo"
version = "0.0.0"
dependencies = [
  "tqdm ==4.66.0",
]
[tool.uv.sources]
tqdm = { url = "§invalid#+#*Ä" }
```

```
error: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 10, column 16
   |
10 | tqdm = { url = "§invalid#+#*Ä" }
   |                ^^^^^^^^^^^^^^^^^
invalid value: string "§invalid#+#*Ä", expected relative URL without a base
```

It says that expected a relative URL without a base, when this was the unexpected input that caused the error. Using `serde::de::Error::custom` oder `serde::de::Error::invalid_value` improves the error message:

```
error: TOML parse error at line 8, column 16
  |
8 | tqdm = { url = "§invalid#+#*Ä" }
  |                ^^^^^^^^^^^^^^^^^
relative URL without a base: "§invalid#+#*Ä"
```

url/src/lib.rs

@@ -2700,7 +2700,7 @@ impl Url {
     where
         D: serde::Deserializer<'de>,
     {
-        use serde::de::{Deserialize, Error, Unexpected};
+        use serde::de::{Deserialize, Error};
         let (
             serialization,
             scheme_end,
@@ -2726,10 +2726,8 @@ impl Url {
             fragment_start,
         };
         if cfg!(debug_assertions) {
-            url.check_invariants().map_err(|reason| {
-                let reason: &str = &reason;
-                Error::invalid_value(Unexpected::Other("value"), &reason)
-            })?
+            url.check_invariants()
+                .map_err(|reason| Error::custom(reason))?
         }
         Ok(url)
     }
@@ -2942,7 +2940,7 @@ impl<'de> serde::Deserialize<'de> for Url {
     where
         D: serde::Deserializer<'de>,
     {
-        use serde::de::{Error, Unexpected, Visitor};
+        use serde::de::{Error, Visitor};
 
         struct UrlVisitor;
 
@@ -2957,10 +2955,7 @@ impl<'de> serde::Deserialize<'de> for Url {
             where
                 E: Error,
             {
-                Url::parse(s).map_err(|err| {
-                    let err_s = format!("{}", err);
-                    Error::invalid_value(Unexpected::Str(s), &err_s.as_str())
-                })
+                Url::parse(s).map_err(|err| Error::custom(format!("{}: {:?}", err, s)))
             }
         }
 

url/tests/unit.rs

@@ -1362,3 +1362,20 @@ fn issue_974() {
     let _ = url::quirks::set_port(&mut url, "\u{0000}9000");
     assert_eq!(url.port(), Some(8000));
 }
+
+#[cfg(feature = "serde")]
+#[test]
+fn serde_error_message() {
+    use serde::Deserialize;
+    #[derive(Debug, Deserialize)]
+    #[allow(dead_code)]
+    struct TypeWithUrl {
+        url: Url,
+    }
+
+    let err = serde_json::from_str::<TypeWithUrl>(r#"{"url": "§invalid#+#*Ä"}"#).unwrap_err();
+    assert_eq!(
+        err.to_string(),
+        r#"relative URL without a base: "§invalid#+#*Ä" at line 1 column 25"#
+    );
+}