Deprecate internal error generation functions

The functions generating errors in the executor should be changed anyway
when we provide better error messages from the executor, with the error
location and response path. So public definitions of these functions are
deprecated now and they are replaced by more generic functions in the
executor code.

CHANGELOG.md

@@ -23,6 +23,8 @@ and this project adheres to
 ### Changed
 - `AST.Document.Value.List` and `AST.Document.ConstValue.ConstList` contain
   location information for each list item.
+- `Error`: `singleError`, `addErr` and `addErrMsg` are deprecated. They are
+  internal functions used by the executor for error handling.
 
 ## [0.11.1.0] - 2021-02-07
 ### Added