Better error when plugins don't call the send function

CHANGES.org

@@ -1,3 +1,11 @@
+* 1.0+beta5
+
+- When =ocamlfind= is present in the =PATH=, do not attempt to call
+ =opam config var lib=
+
+- Better error message when a jbuild file in OCaml syntax forgets to
+  call =Jbuild_plugin.V*.send=
+
 * 1.0+beta4 (20/03/2017)
 
 - Improve error messages about invalid/missing pkg.opam files

src/jbuild_load.ml

@@ -118,6 +118,10 @@ end
           (Path.to_string context.ocaml)
           args
         >>= fun () ->
+        if not (Path.exists generated_jbuild) then
+          die "@{<error>Error:@} %s failed to produce a valid jbuild file.\n\
+               Did you forgot to call [Jbuild_plugin.V*.send]?"
+            (Path.to_string file);
         let sexps = Sexp_load.many (Path.to_string generated_jbuild) in
         return (dir, Stanzas.parse sexps ~dir ~visible_packages ~closest_packages))
     |> Future.all