added functionality for printing missing templates when rendering manifest.yml files

src/packme/PackerTemplate.py

 import collections
 import glob
 import jinja2
+import jinja2.meta
 import json
 import os
 import pprint
+import sys
 import yaml
 
 from typing import Dict, List
@@ -270,12 +272,24 @@ class PackerTemplate:
         node["provisioners"] = self._provisioners
         node["post-processors"] = self._postprocessors
 
-        # Render the jinja2 templates with the parameters dictionary provided in the template file and the available environment variables
-        jinja_template = jinja2.Template(repr(node))
-        s = jinja_template.render(environment=self._environment)
+        env = jinja2.Environment(undefined=jinja2.DebugUndefined)
+        template = env.from_string(repr(node))
+        rendered = template.render(**self._environment)
+
+        # Check if rendering was done correctly
+        ast = env.parse(rendered)
+        undefined = jinja2.meta.find_undeclared_variables(ast)
+        if undefined:
+            print(f'The following variables are undefined: {undefined!r}')
+            sys.exit(1)
+
+        # Replace back %< .* %> to {{ .* }}
+        rendered = rendered.replace("%<","{{")
+        rendered = rendered.replace(">%","}}")
+
         # Dump to the output file
         with open(output_file, "w") as fout:
-            json.dump(yaml.safe_load(s), fout, **kwargs)
+            json.dump(yaml.safe_load(rendered), fout, **kwargs)
 
     def __str__(self) -> str:
         """Returns the string representation for this :class:`PackerTemplate`.