Use ~message and ~here in Option.value_exn

The ~here is set to the point of usage of Option.value_exn itself to
conform with our code base's current practices, but we might want to
have it passed in from the caller of these functions.

Author: cjjdespres

src/lib/merkle_ledger/database.ml

@@ -282,7 +282,9 @@ module Make (Inputs : Intf.Inputs.DATABASE) = struct
     let addr = Addr.of_int_exn ~ledger_depth:mdb.depth index in
     get mdb (Location.Account addr)
 
-  let get_at_index_exn mdb index = get_at_index mdb index |> Option.value_exn
+  let get_at_index_exn mdb index =
+    get_at_index mdb index
+    |> Option.value_exn ~message:"Expected account at index" ~here:[%here]
 
   let all_accounts (t : t) =
     match Account_location.last_location_address t with
@@ -584,7 +586,9 @@ module Make (Inputs : Intf.Inputs.DATABASE) = struct
 
   let iteri t ~f =
     iteri_untrusted t ~f:(fun index account_opt ->
-        f index (Option.value_exn account_opt) )
+        f index
+          (Option.value_exn ~message:"Expected account at index" ~here:[%here]
+             account_opt ) )
 
   (* TODO : if key-value store supports iteration mechanism, like RocksDB,
      maybe use that here, instead of loading all accounts into memory See Issue

src/lib/merkle_mask/masking_merkle_tree.ml

@@ -886,7 +886,8 @@ module Make (Inputs : Inputs_intf.S) = struct
 
     let get_at_index_exn t index =
       assert_is_attached t ;
-      get_at_index t index |> Option.value_exn
+      get_at_index t index
+      |> Option.value_exn ~message:"Expected account at index" ~here:[%here]
 
     let set_at_index_exn t index account =
       assert_is_attached t ;
@@ -918,7 +919,9 @@ module Make (Inputs : Inputs_intf.S) = struct
 
     let iteri t ~f =
       iteri_untrusted t ~f:(fun index account_opt ->
-          f index (Option.value_exn account_opt) )
+          f index
+            (Option.value_exn ~message:"Expected account at index" ~here:[%here]
+               account_opt ) )
 
     let foldi_with_ignored_accounts t ignored_accounts ~init ~f =
       assert_is_attached t ;