system / fix flashmessenger warning / cannot mix flashMessenger() args

https://github.com/pi-engine/pi/commit/f57f86003126f5256b5b6a0280a9b7978e28032b
: improved code readability but add a regression on message display.
In the same class, can not mix error and warning  :  it breaks message
display => i revert from "warning" to "error" the flashMessenger() arg,
to have consistency with other security checks.