Gracefully handle FileNotFoundError exceptions

tests/test_flush.py

@@ -52,3 +52,19 @@ def test_wrong_method(client, mocker):
                           headers={"Authorization": "Basic " + valid_credentials})
 
     assert response.status_code == 405
+
+
+def test_filenotfound(app, mocker):
+
+    app.config.update({
+        "AUTHFILE": '../tests/nonexistent-users-test.txt'
+    })
+
+    client = app.test_client()
+
+    valid_credentials = base64.b64encode(b"test.example.com:testpassword").decode("utf-8")
+    name = 'nonexistent'
+    response = client.get(f"/flush/{name}",
+                          headers={"Authorization": "Basic " + valid_credentials})
+
+    assert response.status_code == 500