Throw an error when an import alias references an invalid macro (#558)

pebble/src/main/java/com/mitchellbosecke/pebble/template/PebbleTemplateImpl.java

@@ -244,6 +244,12 @@ public void importNamedMacrosFromTemplate(String name, List<Pair<String, String>
         .getTemplate(this.resolveRelativePath(name));
     for (Pair<String, String> pair : namedMacros) {
       Macro m = templateImpl.macros.get(pair.getRight());
+
+      if (m == null) {
+        throw new PebbleException(null, "Function or Macro [" + pair.getRight() + "] referenced by alias ["
+                + pair.getLeft() + "] does not exist.");
+      }
+
       this.registerMacro(pair.getLeft(), m);
     }
   }

pebble/src/test/java/com/mitchellbosecke/pebble/macro/TestMacroCalls.java

@@ -161,4 +161,22 @@ void testInvalidSameAliasMacroWithImportAsToken() throws IOException {
           .startsWith("More than one named template can not share the same name"));
     }
   }
+
+  @Test
+  void testInvalidAliasReferencingUnknownMacro() throws IOException {
+    PebbleEngine pebble = new PebbleEngine.Builder().build();
+
+    try {
+      PebbleTemplate template = pebble
+          .getTemplate("templates/macros/invalid.from.unknownMacro.peb");
+      Writer writer = new StringWriter();
+      template.evaluate(writer);
+      fail("expected PebbleException");
+    } catch (PebbleException e) {
+      assertEquals(
+              "Function or Macro [iDontExist] referenced by alias [macro_test] does not exist.",
+              e.getPebbleMessage()
+      );
+    }
+  }
 }

pebble/src/test/resources/templates/macros/invalid.from.unknownMacro.peb

@@ -0,0 +1,4 @@
+Hello
+
+{% from "templates/macros/macro.peb" import iDontExist as macro_test %}
+Call 1: {{ macro_test() }}