Fix ClassNotFoundException when vault is missing (#4151)

Essentials/src/main/java/com/earth2me/essentials/Essentials.java

@@ -207,8 +207,13 @@ public void setupForTesting(final Server server) throws IOException, InvalidDesc
 
     @Override
     public void onLoad() {
-        // Vault registers their Essentials provider at low priority, so we have to use normal priority here
-        getServer().getServicesManager().register(net.milkbowl.vault.economy.Economy.class, new VaultEconomyProvider(this), this, ServicePriority.Normal);
+        try {
+            // Vault registers their Essentials provider at low priority, so we have to use normal priority here
+            Class.forName("net.milkbowl.vault.economy.Economy");
+            getServer().getServicesManager().register(net.milkbowl.vault.economy.Economy.class, new VaultEconomyProvider(this), this, ServicePriority.Normal);
+        } catch (final ClassNotFoundException ignored) {
+            // Probably safer than fetching for the plugin as bukkit may not have marked it as enabled at this point in time
+        }
     }
 
     @Override