improve config.toml error msg (#1155)

improve config.toml error msg

when I write a unvalid config.toml
```toml
[[sources]]
# name = "company-internal"
url = "https://company.internal/simple/"
```
the before msg is very strange: `error: expected name`
after: `error: bad config.toml syntax,expected source.name`

rye/src/config.rs

@@ -232,7 +232,8 @@ impl Config {
         if let Some(sources) = self.doc.get("sources").map(|x| toml::iter_tables(x)) {
             for source in sources {
                 let source = source.context("invalid value for source in config.toml")?;
-                let source_ref = SourceRef::from_toml_table(source)?;
+                let source_ref = SourceRef::from_toml_table(source)
+                    .context("invalid source definition in config.toml")?;
                 if source_ref.name == "default" {
                     need_default = false;
                 }

rye/src/pyproject.rs

@@ -152,12 +152,12 @@ impl SourceRef {
             .get("name")
             .and_then(|x| x.as_str())
             .map(|x| x.to_string())
-            .ok_or_else(|| anyhow!("expected name"))?;
+            .ok_or_else(|| anyhow!("expected source.name"))?;
         let url = source
             .get("url")
             .and_then(|x| x.as_str())
             .map(|x| x.to_string())
-            .ok_or_else(|| anyhow!("expected url"))?;
+            .ok_or_else(|| anyhow!("expected source.url"))?;
         let verify_ssl = source
             .get("verify_ssl")
             .and_then(|x| x.as_bool())
@@ -174,7 +174,7 @@ impl SourceRef {
             .get("type")
             .and_then(|x| x.as_str())
             .map_or(Ok(SourceRefType::Index), |x| x.parse::<SourceRefType>())
-            .context("invalid value for type")?;
+            .context("invalid value for source.type")?;
         Ok(SourceRef {
             name,
             url,
@@ -1265,7 +1265,8 @@ fn get_sources(doc: &DocumentMut) -> Result<Vec<SourceRef>, Error> {
     {
         for source in sources {
             let source = source.context("invalid value for pyproject.toml's tool.rye.sources")?;
-            let source_ref = SourceRef::from_toml_table(source)?;
+            let source_ref = SourceRef::from_toml_table(source)
+                .context("invalid source definition in pyproject.toml")?;
             rv.push(source_ref);
         }
     }
@@ -1320,6 +1321,7 @@ fn get_project_metadata(path: &Path) -> Result<Metadata, Error> {
     }
     serde_json::from_slice(&metadata.stdout).map_err(Into::into)
 }
+
 /// Represents expanded sources.
 #[derive(Debug, Clone, Serialize)]
 pub struct ExpandedSources {