Improve password check

1. Fail when the db contains encrypted docs and no password is set.
   Previously, this case wasn't detected.

2. Exit with an error instead of showing warnings.
   This ensures that we never store docs with different encryption passwords.

src/documents/checks.py

@@ -1,5 +1,5 @@
 from django.conf import settings
-from django.core.checks import Warning, register
+from django.core.checks import Error, register
 from django.db.utils import OperationalError
 
 
@@ -9,25 +9,28 @@ def changed_password_check(app_configs, **kwargs):
     from documents.models import Document
     from paperless.db import GnuPG
 
-    if not settings.PASSPHRASE:
-        return []
-
-    warning = (
-        "At least one document:\n\n  {}\n\nin your data store was encrypted "
-        "with a password other than the one currently\nin use.  This means "
-        "that this file, and others encrypted with the other\npassword are no "
-        "longer acessible, which is probably not what you want.  If\nyou "
-        "intend to change your Paperless password, you must first export all "
-        "of\nthe old documents, start fresh with the new password and then "
-        "re-import them."
-    )
-
     try:
-        document = Document.objects.order_by("-pk").filter(
+        encrypted_doc = Document.objects.filter(
             storage_type=Document.STORAGE_TYPE_GPG).first()
-        if document and not GnuPG.decrypted(document.source_file):
-            return [Warning(warning.format(document))]
     except OperationalError:
-        pass  # No documents table yet
+        return []  # No documents table yet
+
+    if encrypted_doc:
+        if not settings.PASSPHRASE:
+            return [Error(
+                "The database contains encrypted documents but no password "
+                "is set."
+            )]
+        elif not GnuPG.decrypted(encrypted_doc.source_file):
+            import textwrap
+            return [Error(textwrap.dedent(
+                """
+                The current password doesn't match the password of the
+                existing documents.
+
+                If you intend to change your password, you must first export
+                all of the old documents, start fresh with the new password
+                and then re-import them."
+                """))]
 
     return []