Use a better exception if Popup.Feedback does not get a block

Before: NoMethodError: undefined method `call' for nil:NilClass
After:  ArgumentError: block must be supplied

This fixes a RSpec warning about testing a generic exception:
Using the `raise_error` matcher without providing a specific error or message

library/general/src/modules/Popup.rb

@@ -1068,6 +1068,8 @@ def ShowFeedback(headline, message)
     # @param message [String] message with details, displayed below the headline
     # @param block block to execute
     def Feedback(headline, message, &block)
+      raise ArgumentError, "block must be supplied" unless block
+
       ShowFeedback(headline, message)
       block.call
     ensure

library/general/test/popup_test.rb

@@ -14,26 +14,30 @@
   end
 
   describe ".Feedback" do
-    before do
-      expect(ui).to receive(:OpenDialog)
-      expect(ui).to receive(:CloseDialog)
-      allow(ui).to receive(:BusyCursor)
-      allow(ui).to receive(:GetDisplayInfo).and_return({})
+    context "when arguments are good" do
+      before do
+        expect(ui).to receive(:OpenDialog)
+        expect(ui).to receive(:CloseDialog)
+        allow(ui).to receive(:BusyCursor)
+        allow(ui).to receive(:GetDisplayInfo).and_return({})
+      end
+
+      it "opens a popup dialog and closes it at the end" do
+        # just pass an empty block
+        subject.Feedback("Label", "Message") {}
+      end
+
+      it "closes the popup even when an exception occurs in the block" do
+        # raise an exception in the block
+        expect { subject.Feedback("Label", "Message") { raise "TEST" } }.to raise_error(RuntimeError, "TEST")
+      end
     end
 
-    it "opens a popup dialog and closes it at the end" do
-      # just pass an empty block
-      subject.Feedback("Label", "Message") {}
-    end
-
-    it "closes the popup even when an exception occurs in the block" do
-      # raise an exception in the block
-      expect { subject.Feedback("Label", "Message") { raise "TEST" } }.to raise_error(RuntimeError, "TEST")
-    end
-
-    it "raises exception when the block parameter is missing" do
-      # no block passed
-      expect { subject.Feedback("Label", "Message") }.to raise_error
+    context "when arguments are bad" do
+      it "raises exception when the block parameter is missing" do
+        # no block passed
+        expect { subject.Feedback("Label", "Message") }.to raise_error(ArgumentError, /block must be supplied/)
+      end
     end
   end