mission_base: fix to set the end of mission item on error

[KonradRudin]

Solved Problem

The UAV could make a fly away, if a mission could not be started but the mission was still declared valid.

Solution

• Make sure to properly set a loiter item or idle item, when a mission can't be started or the current mission item could not be loaded

Changelog Entry

For release notes:

Bugfix: Prevent flyaway when an in air mission could not be started and the mission item could not be properly loaded

Test coverage

• Simulation test: Tested on gazebo-classic_standard_vtol