Make 'return' a reserved (but invalid) keyword

src/aeso_parse_lib.erl

@@ -355,6 +355,7 @@ unexpected_token_error(Ts, Expect, T) ->
               {con, _, X}   when ExpectId               -> io_lib:format(" Did you mean ~s?", [mk_lower(X)]);
               {qcon, _, Xs} when ExpectCon              -> io_lib:format(" Did you mean ~s?", [lists:last(Xs)]);
               {qid, _,  Xs} when ExpectId               -> io_lib:format(" Did you mean ~s?", [lists:last(Xs)]);
+              {return, _} -> " [Polite reminder that Sophia is not JavaScript]";
               _ -> ""
           end,
     mk_error(Ts, io_lib:format("Unexpected ~s.~s", [describe(T), Fix])).

src/aeso_scan.erl

@@ -37,7 +37,8 @@ lexer() ->
         , {"[^/*]+|[/*]", skip()} ],
 
     Keywords = ["contract", "include", "let", "switch", "type", "record", "datatype", "if", "elif", "else", "function",
-                "stateful", "payable", "true", "false", "mod", "public", "entrypoint", "private", "indexed", "namespace"],
+                "stateful", "payable", "true", "false", "mod", "public", "entrypoint", "private", "indexed", "namespace",
+                "return"],
     KW = string:join(Keywords, "|"),
 
     Rules =