Improve error message when subclassing non-Class

Fixes [Bug #14726]

class.c

@@ -227,7 +227,7 @@ void
 rb_check_inheritable(VALUE super)
 {
     if (!RB_TYPE_P(super, T_CLASS)) {
-	rb_raise(rb_eTypeError, "superclass must be a Class (%"PRIsVALUE" given)",
+        rb_raise(rb_eTypeError, "superclass must be an instance of Class (given an instance of %"PRIsVALUE")",
 		 rb_obj_class(super));
     }
     if (RBASIC(super)->flags & FL_SINGLETON) {

spec/ruby/core/class/new_spec.rb

@@ -95,7 +95,7 @@ describe "Class.new" do
   end
 
   it "raises a TypeError when given a non-Class" do
-    error_msg = /superclass must be a Class/
+    error_msg = /superclass must be a.*Class/
     -> { Class.new("")              }.should raise_error(TypeError, error_msg)
     -> { Class.new(1)               }.should raise_error(TypeError, error_msg)
     -> { Class.new(:symbol)         }.should raise_error(TypeError, error_msg)

spec/ruby/language/class_spec.rb

@@ -286,7 +286,7 @@ describe "A class definition extending an object (sclass)" do
   end
 
   it "raises a TypeError when trying to extend non-Class" do
-    error_msg = /superclass must be a Class/
+    error_msg = /superclass must be a.* Class/
     -> { class TestClass < "";              end }.should raise_error(TypeError, error_msg)
     -> { class TestClass < 1;               end }.should raise_error(TypeError, error_msg)
     -> { class TestClass < :symbol;         end }.should raise_error(TypeError, error_msg)

vm_insnhelper.c

@@ -4068,7 +4068,7 @@ vm_define_class(ID id, rb_num_t flags, VALUE cbase, VALUE super)
 
     if (VM_DEFINECLASS_HAS_SUPERCLASS_P(flags) && !RB_TYPE_P(super, T_CLASS)) {
 	rb_raise(rb_eTypeError,
-		 "superclass must be a Class (%"PRIsVALUE" given)",
+                 "superclass must be an instance of Class (given an instance of %"PRIsVALUE")",
 		 rb_obj_class(super));
     }