#1497 Show specific message for invalid YAML files (#1506)

* #1497 Throw dedicated exception for invalid YAML files and handle it on startup
- Wrap SnakeYAML exceptions when loading config.yml and commands.yml on startup into own exception type
- Handle exception type on startup with specific error message

* #1497 Fix inaccurate JavaDoc comment

src/main/java/fr/xephi/authme/util/ExceptionUtils.java

@@ -0,0 +1,36 @@
+package fr.xephi.authme.util;
+
+import com.google.common.collect.Sets;
+
+import java.util.Set;
+
+/**
+ * Utilities for exceptions.
+ */
+public final class ExceptionUtils {
+
+    private ExceptionUtils() {
+    }
+
+    /**
+     * Returns the first throwable of the given {@code wantedThrowableType} by visiting the provided
+     * throwable and its causes recursively.
+     *
+     * @param wantedThrowableType the throwable type to find
+     * @param throwable the throwable to start with
+     * @param <T> the desired throwable subtype
+     * @return the first throwable found of the given type, or null if none found
+     */
+    public static <T extends Throwable> T findThrowableInCause(Class<T> wantedThrowableType, Throwable throwable) {
+        Set<Throwable> visitedObjects = Sets.newIdentityHashSet();
+        Throwable currentThrowable = throwable;
+        while (currentThrowable != null && !visitedObjects.contains(currentThrowable)) {
+            if (wantedThrowableType.isInstance(currentThrowable)) {
+                return wantedThrowableType.cast(currentThrowable);
+            }
+            visitedObjects.add(currentThrowable);
+            currentThrowable = currentThrowable.getCause();
+        }
+        return null;
+    }
+}