Emit warnings when safe_eval() raises a SyntaxError or other Exception (#14304)

This change is related to reported issue #14291 and pull request #14293.

Without the fix from #14293, this change will emit a warning as shown below, on the following playbook:

``yaml
---
- hosts: localhost
  gather_facts: no
  vars:
    works:
      key1: 'string'
      key2: 1234
    fails:
      key1: 'string'
      key2: 1234
      key3: false
  tasks:
  - debug: msg={{ works | to_json }}
  - debug: msg={{ fails | to_json }}
```

On error, this results in a proper warning:

```
[dag@moria ansible.dag]$ ansible-playbook test49.yml

PLAY ***************************************************************************

TASK [debug] *******************************************************************
ok: [localhost] => {
    "msg": {
        "key1": "string",
        "key2": 1234
    }
}

TASK [debug] *******************************************************************
 [WARNING]: Error in expression "{"key3": false, "key2": 1234, "key1": "string"}". (name 'false' is not defined)

ok: [localhost] => {
    "msg": "{\"key3\": false, \"key2\": 1234, \"key1\": \"string\"}"
}

PLAY RECAP *********************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0
```

lib/ansible/template/safe_eval.py

@@ -26,6 +26,12 @@ from ansible.compat.six.moves import builtins
 from ansible import constants as C
 from ansible.plugins import filter_loader, test_loader
 
+try:
+    from __main__ import display
+except ImportError:
+    from ansible.utils.display import Display
+    display = Display()
+
 def safe_eval(expr, locals={}, include_exceptions=False):
     '''
     This is intended for allowing things like:
@@ -131,13 +137,14 @@ def safe_eval(expr, locals={}, include_exceptions=False):
         else:
             return result
     except SyntaxError as e:
+        display.warning('SyntaxError in safe_eval() on expr: %s (%s)' % (expr, e))
         # special handling for syntax errors, we just return
         # the expression string back as-is
         if include_exceptions:
             return (expr, None)
         return expr
     except Exception as e:
+        display.warning('Exception in safe_eval() on expr: %s (%s)' % (expr, e))
         if include_exceptions:
             return (expr, e)
         return expr
-