More explicit error message when corpus_id is not set in environment

To avoid the ugly 500 errors that env get when they try to use API endpoints with corpus_id=None.

There already was a warning but this one is more explicit.

arkindex_worker/worker/__init__.py

@@ -15,7 +15,6 @@ from apistar.exceptions import ErrorResponse
 from arkindex_worker import logger
 from arkindex_worker.cache import CachedElement
 from arkindex_worker.models import Element
-from arkindex_worker.reporting import Reporter
 from arkindex_worker.worker.base import BaseWorker
 from arkindex_worker.worker.classification import ClassificationMixin
 from arkindex_worker.worker.element import ElementMixin
@@ -156,17 +155,12 @@ class ElementsWorker(
             super().configure()
             super().configure_cache()
 
-        # Add report concerning elements
-        self.report = Reporter(
-            **self.worker_details, version=getattr(self, "worker_version_id", None)
-        )
-
     def run(self):
         """
         Implements an Arkindex worker that goes through each element returned by
-        [list_elements][arkindex_worker.worker.ElementsWorker.list_elements]. It calls [process_element][arkindex_worker.worker.ElementsWorker.process_element], catching exceptions
-        and reporting them using the [Reporter][arkindex_worker.reporting.Reporter], and handles saving the report
-        once the process is complete as well as WorkerActivity updates when enabled.
+        [list_elements][arkindex_worker.worker.ElementsWorker.list_elements].
+        It calls [process_element][arkindex_worker.worker.ElementsWorker.process_element],
+        catching exceptions, and handles saving WorkerActivity updates when enabled.
         """
         self.configure()
 
@@ -232,10 +226,6 @@ class ElementsWorker(
                         self.update_activity(element.id, ActivityState.Error)
                     except Exception:
                         pass
-                self.report.error(element_id, e)
-
-        # Save report as local artifact
-        self.report.save(self.work_dir / "ml_report.json")
 
         if failed:
             logger.error(