[Feature] - Display proper error when key is missing

[Zoriot]

    java.lang.IndexOutOfBoundsException: Index 6 out of bounds for length 6
    at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:100) ~[?:?]
    at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:106) ~[?:?]
    at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:302) ~[?:?]
    at java.base/java.util.Objects.checkIndex(Objects.java:385) ~[?:?]
    at java.base/java.util.ArrayList.get(ArrayList.java:427) ~[?:?]
    at PlotSystem-4.1.4-.jar/com.alpsbte.plotsystem.core.system.tutorial.BeginnerTutorial$Stage10.getTimeline(BeginnerTutorial.java:636) ~[PlotSystem-4.1.4-.jar:?]
    at PlotSystem-4.1.4-.jar/com.alpsbte.plotsystem.core.system.tutorial.AbstractTutorial.nextStage(AbstractTutorial.java:225) ~[PlotSystem-4.1.4-.jar:?]

Currently you get a error like that when a key is missing. We should display a proper error message, when there is a missing segment in the documentation-links part of the tutorial config, also other segments should be checked too → think it mainly effects arrays where multiple values are. Other possibility is just to handle it silently and dont use the segment.