Invalid materials in challenge yml

[Muspah]

Handle this in a nicer way then throwing exceptions at the user.

[14:05:38] [Server thread/ERROR]: Error occurred while enabling uSkyBlock v2.8.7 (Is it up to date?) java.lang.IllegalArgumentException: Bukkit 1.13 does not know the material GOLD_HORSE_ARMOR! at us.talabrek.ultimateskyblock.utils.util.ItemStackUtil.getItemType(ItemStackUtil.java:68) ~[?:?] at us.talabrek.ultimateskyblock.utils.util.ItemStackUtil.createItemsWithProbabilty(ItemStackUtil.java:42) ~[?:?] at us.talabrek.ultimateskyblock.challenge.ChallengeFactory.createReward(ChallengeFactory.java:113) ~[?:?] at us.talabrek.ultimateskyblock.challenge.ChallengeFactory.createChallenge(ChallengeFactory.java:64) ~[?:?] at us.talabrek.ultimateskyblock.challenge.Rank.(Rank.java:31) ~[?:?] at us.talabrek.ultimateskyblock.challenge.ChallengeFactory.createRankMap(ChallengeFactory.java:125) ~[?:?] at us.talabrek.ultimateskyblock.challenge.ChallengeLogic.(ChallengeLogic.java:70) ~[?:?] at us.talabrek.ultimateskyblock.uSkyBlock.reloadConfigs(uSkyBlock.java:805) ~[?:?] at us.talabrek.ultimateskyblock.uSkyBlock.onEnable(uSkyBlock.java:209) ~[?:?] at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:263) ~[spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:351) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:480) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.craftbukkit.v1_15_R1.CraftServer.enablePlugin(CraftServer.java:464) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.craftbukkit.v1_15_R1.CraftServer.enablePlugins(CraftServer.java:378) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:211) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:784) [spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at java.lang.Thread.run(Thread.java:813) [?:1.8.0_212] Caused by: java.lang.NullPointerException at org.bukkit.craftbukkit.v1_15_R1.legacy.CraftLegacy.fromLegacyData(CraftLegacy.java:95) ~[spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.craftbukkit.v1_15_R1.util.CraftMagicNumbers.getBlock(CraftMagicNumbers.java:59) ~[spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at org.bukkit.craftbukkit.v1_15_R1.util.CraftMagicNumbers.fromLegacy(CraftMagicNumbers.java:163) ~[spigot-1.15.2.jar:git-Spigot-2040c4c-933e909] at us.talabrek.ultimateskyblock.utils.util.ItemStackUtil.getItemType(ItemStackUtil.java:63) ~[?:?] ... 16 more