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.

jellyseerr/models.go

@@ -130,3 +130,7 @@ type MainUserSettings struct {
 	WatchlistSyncMovies any    `json:"watchlistSyncMovies,omitempty"`
 	WatchlistSyncTv     any    `json:"watchlistSyncTv,omitempty"`
 }
+
+type ErrorDTO struct {
+	Message string `json:"message,omitempty"`
+}