jellyseerr: cleanup requests method, read proper errors

single req() function is wrapped by methods for each http method, and
error messages are parsed and returned if given by the server.

also added note about Jellyseerr's enforcement of unique email addresses
in settings.

config/config-base.json

@@ -1629,6 +1629,14 @@
                     "value": false,
                     "depends_true": "enabled",
                     "description": "Existing users (and those created outside jfa-go) will have their contact info imported to Jellyseerr."
+                },
+                "constraints_note": {
+                    "name": "Unique Emails:",
+                    "type": "note",
+                    "value": "",
+                    "depends_true": "import_existing",
+                    "required": "false",
+                    "description": "Jellyseerr requires email addresses to be unique. If this is not the case, you may see errors in jfa-go's logs. You can require unique addresses in Settings > Email."
                 }
             }
         },