add an Error exception to ast.ml so we have something that can be raised from anywhere

ast.ml

@@ -437,6 +437,8 @@ type type_decl = type_def * pos
 
 type package = string list * type_decl list
 
+exception Error of string * pos
+
 let is_lower_ident i =
 	let rec loop p =
 		match String.unsafe_get i p with

main.ml

@@ -1589,6 +1589,8 @@ try
 with
 	| Abort ->
 		()
+	| Ast.Error (m,p) ->
+		error ctx m p
 	| Typecore.Fatal_error (m,p) ->
 		error ctx m p
 	| Common.Abort (m,p) ->