Tweaking error message resulting from undefined template variables

Fixes #5114

lib/ansible/utils/template.py

@@ -338,7 +338,10 @@ def template_from_string(basedir, data, vars, fail_on_undefined=False):
             res = jinja2.utils.concat(rf)
         except TypeError, te:
             if 'StrictUndefined' in str(te):
-                raise errors.AnsibleUndefinedVariable("unable to look up a name or access an attribute in template string")
+                raise errors.AnsibleUndefinedVariable(
+                    "Unable to look up a name or access an attribute in template string. " + \
+                    "Make sure your variable name does not contain invalid characters like '-'."
+                )
             else:
                 raise errors.AnsibleError("an unexpected type error occured. Error was %s" % te)
         return res