Added support for soft dependencies.

Soft dependencies allow plugins to request to be loaded after another plugin, but they will not throw an UnknownDependency exception if the other plugin is not present.

By: Raphfrk <raphfrk@gmail.com>

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

@@ -0,0 +1,45 @@
+package org.bukkit.plugin;
+
+/**
+ * Thrown when attempting to load an invalid Plugin file
+ */
+public class UnknownSoftDependencyException extends UnknownDependencyException {
+
+    private static final long serialVersionUID = 5721389371901775899L;
+
+    /**
+     * Constructs a new UnknownSoftDependencyException based on the given Exception
+     *
+     * @param throwable Exception that triggered this Exception
+     */
+    public UnknownSoftDependencyException(Throwable throwable) {
+        this(throwable, "Unknown soft dependency");
+    }
+
+    /**
+     * Constructs a new UnknownSoftDependencyException with the given message
+     *
+     * @param message Brief message explaining the cause of the exception
+     */
+    public UnknownSoftDependencyException(final String message) {
+        this(null, message);
+    }
+
+    /**
+     * Constructs a new UnknownSoftDependencyException based on the given Exception
+     *
+     * @param message Brief message explaining the cause of the exception
+     * @param throwable Exception that triggered this Exception
+     */
+    public UnknownSoftDependencyException(final Throwable throwable, final String message) {
+        super(throwable, message);
+    }
+
+    /**
+     * Constructs a new UnknownSoftDependencyException
+     */
+    public UnknownSoftDependencyException() {
+        this(null, "Unknown dependency");
+    }
+
+}