SPIGOT-5783: Add helpful info to UnknownDependencyException

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -266,7 +266,7 @@ public final class SimplePluginManager implements PluginManager {
                             server.getLogger().log(
                                 Level.SEVERE,
                                 "Could not load '" + entry.getValue().getPath() + "' in folder '" + directory.getPath() + "'",
-                                new UnknownDependencyException(dependency));
+                                new UnknownDependencyException("Unknown dependency " + dependency + ". Please download and install " + dependency + " to run this plugin."));
                             break;
                         }
                     }

paper-api/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java

@@ -122,7 +122,7 @@ public final class JavaPluginLoader implements PluginLoader {
             Plugin current = server.getPluginManager().getPlugin(pluginName);
 
             if (current == null) {
-                throw new UnknownDependencyException(pluginName);
+                throw new UnknownDependencyException("Unknown dependency " + pluginName + ". Please download and install " + pluginName + " to run this plugin.");
             }
         }