Commander: trigger warning when arming denied due to check failure

[sfuhrer]

Solved Problem

There is currently no warning/event sent by PX4 when arming is denied due to a preflight check failure (though the preflight field in the top bar is red).

Solution

Add a warning message to make clear that

1. there was an arming attempt (not that e.g. sticks are not working)
2. give hint to why arming failed

Changelog Entry

For release notes:

Feature: