Tweak DSL error formatting

- Add a blank line between the DslFilesException stack trace and
   formatted error messages.
 - Quote the sub-function name in the message for UndefinedSubFunction
   errors.

core/src/main/java/it/cavallium/warppi/math/rules/RulesManager.java

@@ -41,6 +41,7 @@ public class RulesManager {
 		} catch (IOException | DslFilesException e) {
 			e.printStackTrace();
 			if (e instanceof DslFilesException) {
+				System.err.println();
 				System.err.print(((DslFilesException) e).format());
 			}
 			Engine.getPlatform().exit(1);

core/src/main/java/it/cavallium/warppi/math/rules/dsl/errorutils/DslErrorMessageFormatter.java

@@ -31,7 +31,7 @@ public class DslErrorMessageFormatter implements DslError.Visitor<String> {
 	@Override
 	public String visit(final UndefinedSubFunction undefinedSubFunction) {
 		return String.format(
-				"Sub-function %s is used in a replacement pattern,\nbut not defined in the target pattern",
+				"Sub-function \"%s\" is used in a replacement pattern,\nbut not defined in the target pattern",
 				undefinedSubFunction.getName()
 		);
 	}