Improve error message for %{<}

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/pform.ml

@@ -53,7 +53,10 @@ module Map = struct
     ; "project_root", since ~version:(1, 0) Var.Project_root
 
     ; "<", deleted_in Var.Deps ~version:(1, 0)
-             ~repl:"Use a named dependency instead: (:<name> <dep>)"
+             ~repl:"Use a named dependency instead:\
+                    \n\
+                    \n\  (deps (:x <dep>) ...)\
+                    \n\   ... %{x} ..."
     ; "@", renamed_in ~version:(1, 0) ~new_name:"targets"
     ; "^", renamed_in ~version:(1, 0) ~new_name:"deps"
     ; "SCOPE_ROOT", renamed_in ~version:(1, 0) ~new_name:"project_root"

test/blackbox-tests/test-cases/syntax-versioning/run.t

@@ -21,6 +21,9 @@
   $ dune build
   File "dune", line 1, characters 40-42:
   Error: %{<} was deleted in version 1.0 of the dune language.
-  Use a named dependency instead: (:<name> <dep>)
+  Use a named dependency instead:
+  
+    (deps (:x <dep>) ...)
+     ... %{x} ...
   [1]
   $ rm -f dune