preserve underlying block building error (ansible#56465)

fixes ansible#56462

changelogs/fragments/better_task_error.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - display underlying error when reporting an invalid ``tasks:`` block.

lib/ansible/playbook/play.py

@@ -22,6 +22,7 @@
 from ansible import constants as C
 from ansible import context
 from ansible.errors import AnsibleParserError, AnsibleAssertionError
+from ansible.module_utils._text import to_native
 from ansible.module_utils.six import string_types
 from ansible.playbook.attribute import FieldAttribute
 from ansible.playbook.base import Base
@@ -145,7 +146,7 @@ def _load_tasks(self, attr, ds):
         try:
             return load_list_of_blocks(ds=ds, play=self, variable_manager=self._variable_manager, loader=self._loader)
         except AssertionError as e:
-            raise AnsibleParserError("A malformed block was encountered while loading tasks", obj=self._ds, orig_exc=e)
+            raise AnsibleParserError("A malformed block was encountered while loading tasks: %s" % to_native(e), obj=self._ds, orig_exc=e)
 
     def _load_pre_tasks(self, attr, ds):
         '''