Add-on: report which files were missing

After submission, list the missing files on the terminal (if there are
any).

CHANGELOG.md

@@ -7,6 +7,7 @@ bugs in actually-released versions.
 ## 3.0 - in development
 
 - Faster & more accurate progress reporting of file submission.
+- Add-on: report which files were missing after submitting a job. This is reported in the terminal (aka System Console).
 
 
 ## 3.0-beta3 - released 2022-08-31

addon/flamenco/bat/interface.py

@@ -85,6 +85,7 @@ class BatProgress(submodules.progress.Callback):  # type: ignore
     ) -> None:
         if missing_files:
             self._txt("There were %d missing files" % len(missing_files))
+            self._log_missing_files(missing_files)
         else:
             self._txt("Pack of %s done" % output_blendfile.name)
 
@@ -117,6 +118,11 @@ class BatProgress(submodules.progress.Callback):  # type: ignore
         # TODO(Sybren): report missing files in a nice way
         pass
 
+    def _log_missing_files(self, missing_files: typing.Set[Path]) -> None:
+        print("Missing files:")
+        for path in sorted(missing_files):
+            print(f"  - {path}")
+
 
 class PackThread(threading.Thread):
     queue: queue.SimpleQueue[Message]