review the internal schema to allow to create schema without identifier; fix #513

meilisearch-schema/src/error.rs

@@ -6,6 +6,7 @@ pub type SResult<T> = Result<T, Error>;
 #[derive(Debug)]
 pub enum Error {
     FieldNameNotFound(String),
+    IdentifierAlreadyPresent,
     MaxFieldsLimitExceeded,
 }
 
@@ -14,6 +15,7 @@ impl fmt::Display for Error {
         use self::Error::*;
         match self {
             FieldNameNotFound(field) => write!(f, "The field {:?} doesn't exist", field),
+            IdentifierAlreadyPresent => write!(f, "The schema already have an identifier. It's impossible to update it"),
             MaxFieldsLimitExceeded => write!(f, "The maximum of possible reattributed field id has been reached"),
         }
     }