Made a specific id for names.

src/mtterror.rs

@@ -12,11 +12,13 @@ pub enum ErrorID {
     Document(NameType),
     DocumentNameAlreadyExists,
     DocumentNotFound,
+    Field(NameType),
     FieldDataMustBeUnique,
     FieldInvalidNone,
     FieldInvalidType,
     FieldMissingData,
     FieldNameAlreadyExists,
+    FieldTypeExpected(FieldType),
     IndexEntryAlreadyExists,
     InvalidFieldName(Name),
     NameAlreadyExists,