Simplify DependencyGuardListTask execute function

[SimonMarquis]

• add early exit condition to reduce indentation
• throw Exception with all potential errors (not on the first occurence)
• don't create the message String twice

[handstandsam]

Thank you. Let's target this for 0.5.0. I'm hoping this latest fix for 0.4.3 is the end of 0.4.x!

[handstandsam]

Merging, thank your for contributing!