refactor: Report inventory loading errors

Issue #157: #517
PR #568: #568

Co-authored-by: Oleh Prypin <oleh@pryp.in>

src/mkdocstrings/plugin.py

@@ -14,7 +14,6 @@
 
 from __future__ import annotations
 
-import collections
 import functools
 import gzip
 import os
@@ -226,16 +225,17 @@ def on_config(self, config: Config, **kwargs: Any) -> Config:  # noqa: ARG002
 
         config["extra_css"].insert(0, self.css_filename)  # So that it has lower priority than user files.
 
-        self._inv_futures = []
+        self._inv_futures = {}
         if to_import:
             inv_loader = futures.ThreadPoolExecutor(4)
             for handler_name, import_item in to_import:
+                loader = self.get_handler(handler_name).load_inventory
                 future = inv_loader.submit(
                     self._load_inventory,  # type: ignore[misc]
-                    self.get_handler(handler_name).load_inventory,
+                    loader,
                     **import_item,
                 )
-                self._inv_futures.append(future)
+                self._inv_futures[future] = (loader, import_item)
             inv_loader.shutdown(wait=False)
 
         if self.config["watch"]:
@@ -286,9 +286,18 @@ def on_env(self, env: Environment, config: Config, *args: Any, **kwargs: Any) ->
         if self._inv_futures:
             log.debug(f"Waiting for {len(self._inv_futures)} inventory download(s)")
             futures.wait(self._inv_futures, timeout=30)
-            for page, identifier in collections.ChainMap(*(fut.result() for fut in self._inv_futures)).items():
+            results = {}
+            # Reversed order so that pages from first futures take precedence:
+            for fut in reversed(list(self._inv_futures)):
+                try:
+                    results.update(fut.result())
+                except Exception as error:  # noqa: BLE001
+                    loader, import_item = self._inv_futures[fut]
+                    loader_name = loader.__func__.__qualname__
+                    log.error(f"Couldn't load inventory {import_item} through {loader_name}: {error}")  # noqa: TRY400
+            for page, identifier in results.items():
                 config["plugins"]["autorefs"].register_url(page, identifier)
-            self._inv_futures = []
+            self._inv_futures = {}
 
     def on_post_build(
         self,