Improve the returned errors from the facet-search route

milli/src/error.rs

@@ -128,6 +128,16 @@ only composed of alphanumeric characters (a-z A-Z 0-9), hyphens (-) and undersco
         }
     )]
     InvalidSortableAttribute { field: String, valid_fields: BTreeSet<String> },
+    #[error("Attribute `{}` is not filterable. {}",
+        .field,
+        match .valid_fields.is_empty() {
+            true => "This index does not have configured filterable attributes.".to_string(),
+            false => format!("Available filterable attributes are: `{}`.",
+                    valid_fields.iter().map(AsRef::as_ref).collect::<Vec<&str>>().join(", ")
+                ),
+        }
+    )]
+    InvalidSearchFacet { field: String, valid_fields: BTreeSet<String> },
     #[error("Attribute `{}` is not searchable. Available searchable attributes are: `{}{}`.",
         .field,
         .valid_fields.iter().map(AsRef::as_ref).collect::<Vec<&str>>().join(", "),

milli/src/search/mod.rs

@@ -263,8 +263,7 @@ impl<'a> SearchForFacetValue<'a> {
 
         let filterable_fields = index.filterable_fields(rtxn)?;
         if !filterable_fields.contains(&self.facet) {
-            // TODO create a new type of error
-            return Err(UserError::InvalidSortableAttribute {
+            return Err(UserError::InvalidSearchFacet {
                 field: self.facet.clone(),
                 valid_fields: filterable_fields.into_iter().collect(),
             })?;