Raise specific error when an anchor isn't defined

lib/psych/exception.rb

@@ -13,6 +13,13 @@ def initialize
     end
   end
 
+  # Subclasses `BadAlias` for backwards compatibility
+  class AnchorNotDefined < BadAlias
+    def initialize anchor_name
+      super "An alias referenced an unknown anchor: #{anchor_name}"
+    end
+  end
+
   class DisallowedClass < Exception
     def initialize action, klass_name
       super "Tried to #{action} unspecified class: #{klass_name}"

lib/psych/visitors/to_ruby.rb

@@ -323,7 +323,7 @@ def visit_Psych_Nodes_Stream o
       end
 
       def visit_Psych_Nodes_Alias o
-        @st.fetch(o.anchor) { raise BadAlias, "Unknown alias: #{o.anchor}" }
+        @st.fetch(o.anchor) { raise AnchorNotDefined, o.anchor }
       end
 
       private

test/psych/test_hash.rb

@@ -124,7 +124,7 @@ def test_anchor_reuse
     end
 
     def test_raises_if_anchor_not_defined
-      assert_raise(Psych::BadAlias) do
+      assert_raise(Psych::AnchorNotDefined) do
         Psych.unsafe_load(<<~eoyml)
           ---
           foo: &foo

test/psych/test_merge_keys.rb

@@ -117,7 +117,7 @@ def test_missing_merge_key
 bar:
   << : *foo
       eoyml
-      exp = assert_raise(Psych::BadAlias) { Psych.load(yaml, aliases: true) }
+      exp = assert_raise(Psych::AnchorNotDefined) { Psych.load(yaml, aliases: true) }
       assert_match 'foo', exp.message
     end