Emit compiler errors for unknown field attributes

Emit an error if an unknown field attribute is used on a
`salsa::tracked` struct. For example, when changing the `return_ref`
attribute in `ParsedFile` to `returns_ref`, the compiler emits:

    error: unrecognized field attribute
       --> examples/lazy-input/main.rs:164:5
        |
    164 |     #[returns_ref]
        |     ^^^^^^^^^^^^^^

components/salsa-macros/src/salsa_struct.rs

@@ -380,11 +380,20 @@ impl<'s> SalsaField<'s> {
 
         // Scan the attributes and look for the salsa attributes:
         for attr in &field.attrs {
+            let mut found = false;
             for (fa, func) in FIELD_OPTION_ATTRIBUTES {
                 if attr.path().is_ident(fa) {
                     func(attr, &mut result);
+                    found = true;
                 }
             }
+
+            if !found {
+                return Err(syn::Error::new_spanned(
+                    attr,
+                    "unrecognized field attribute",
+                ));
+            }
         }
 
         Ok(result)