Properly wrap objects using json default encoder

Our custom encoder for the to_json filter was simply returning the
object if it was not a HostVars object, leading in some cases to a
TypeError when the data contained an undefined variable. This lead
to an odd error message being propagated up, so we now properly catch
this as an undefined variable error.

Fixes #15610

lib/ansible/playbook/base.py

@@ -31,7 +31,7 @@ from ansible.compat.six import iteritems, string_types
 
 from jinja2.exceptions import UndefinedError
 
-from ansible.errors import AnsibleParserError
+from ansible.errors import AnsibleParserError, AnsibleUndefinedVariable
 from ansible.parsing.dataloader import DataLoader
 from ansible.playbook.attribute import Attribute, FieldAttribute
 from ansible.utils.boolean import boolean
@@ -406,7 +406,7 @@ class Base:
             except (TypeError, ValueError) as e:
                 raise AnsibleParserError("the field '%s' has an invalid value (%s), and could not be converted to an %s."
                         " Error was: %s" % (name, value, attribute.isa, e), obj=self.get_ds())
-            except UndefinedError as e:
+            except (AnsibleUndefinedVariable, UndefinedError) as e:
                 if templar._fail_on_undefined_errors and name != 'name':
                     raise AnsibleParserError("the field '%s' has an invalid value, which appears to include a variable that is undefined."
                             " The error was: %s" % (name,e), obj=self.get_ds())

lib/ansible/plugins/filter/core.py

@@ -68,7 +68,7 @@ class AnsibleJSONEncoder(json.JSONEncoder):
         if isinstance(o, HostVars):
             return dict(o)
         else:
-            return o
+            return json.JSONEncoder.default(o)
 
 def to_yaml(a, *args, **kw):
     '''Make verbose, human readable yaml'''