Custom condition failure messages in Ada 2012

Is there a way to specify a custom error/on failure message for pre- and postconditions, by analogy with Predicate_Failure for predicates? I can't seem to be able to find anything in the official documentation. TIA.

You could use a raise expression (see e.g here) as shown in the example below.

main.adb

pragma Assertion_Policy (Check);

with Ada.Text_IO;
with Ada.Float_Text_IO;

procedure Main is
   
   package TIO renames Ada.Text_IO;
   package FIO renames Ada.Float_Text_IO;
   
   
   function Reciprocal (X : Float) return Float is (1.0 / X)
     with Pre => (X /= 0.0 or else
                    raise Constraint_Error with "X must not be 0.");
   
begin
   FIO.Put (Reciprocal (2.0));
   TIO.New_Line;
   
   FIO.Put (Reciprocal (0.0));
   TIO.New_Line;   
end Main;

output

$ ./obj/main
 5.00000E-01

raised CONSTRAINT_ERROR : X must not be 0.
[2020-07-03 22:20:25] process exited with status 1, elapsed time: 00.32s