Better error reporting if no dependencies are specified.

teapot.rb

@@ -92,10 +92,20 @@ def scope_for_namespace(namespace)
 end
 
 define_generator "xcode-config" do |generator|
+	generator.description = <<-EOF
+		Generate appropriate .xcconfig files for use within Xcode.
+		
+		usage: teapot generate xcode-config [dependencies]
+	EOF
+
 	generator.generate do |*dependency_names|
 		chain = context.dependency_chain(dependency_names)
 		ordered = context.direct_targets(chain.ordered)
 
+		if ordered.size == 0
+			raise GeneratorError.new("Empty dependency list, please specify dependencies.", generator)
+		end
+
 		environment = ordered.last[0].environment_for_configuration(context.configuration)
 
 		substitutions = Substitutions.new