templates can now `raise` exceptions.

src/gen-config.cr

@@ -116,10 +116,11 @@ class GenConfig::Context
 			end
 		end
 
-		#options["port"] = Crinja.function do
-		#	service = (arguments.varargs[0]? || "").to_s.gsub /\//, ':'
-		#	`get-port #{service}`.chomp
-		#end
+		options["raise"] = Crinja.function do
+			message = arguments.varargs.join "\n"
+
+			raise Service::Exception.new "template error: #{message}"
+		end
 
 		# FIXME: Move this to a separate binary?
 		options["random_password"] = Crinja.function do