[coq] [loader] Don't swallow plugin loading errors.

`coq-lsp` plugin loader will now be strict in case of a plugin
failure, the previous loose behavior was more convenient for the
 early releases, but it doesn't make sense now and made things
pretty hard to debug on the Windows installer.

CHANGES.md

@@ -15,6 +15,10 @@
    `--ocamlpath`, `-Q` and `-R` arguments (@ejgallego, #555)
  - Describe findlib status in `Workspace.describe`, which is printed
    in the output windows (@ejgallego, #556)
+ - `coq-lsp` plugin loader will now be strict in case of a plugin
+   failure, the previous loose behavior was more convenient for the
+   early releases, but it doesn't make sense now and made things
+   pretty hard to debug on the Windows installer (@ejgallego, #557)
 
 # coq-lsp 0.1.7: Just-in-time
 -----------------------------

coq/loader.ml

@@ -62,9 +62,12 @@ let map_serlib fl_pkg =
     check_package_exists serlib_name
   else None
 
-(* We are more liberal in the case a SerAPI plugin is not availabe, as the
-   fallbacks are "safe", tho will yield way worse incremental behavior for
-   expressions defined by the plugin *)
+(* We used to be liberal here in the case a SerAPI plugin was not available.
+   This proved to be a bad choice as Coq got confused when plugin loading
+   failed. Par-contre, we now need to make the list in `map_serlib` open, so
+   plugins can register whether they support serialization. I'm afraid that'll
+   have to happen via the finlib database as we cannot load anticipatedly a
+   plugin that may not exist. *)
 let safe_loader loader fl_pkg =
   try loader [fl_pkg]
   with
@@ -74,7 +77,7 @@ let safe_loader loader fl_pkg =
      Feedback.msg_warning
       Pp.(str "Loading findlib plugin: " ++ str fl_pkg
           ++ str "failed" ++ fnl () ++ exn_msg);
-    ()
+    Exninfo.iraise iexn
 
 let plugin_handler user_loader =
   let loader = Option.default (Fl_dynload.load_packages ~debug:false) user_loader in