Don't just raise AnsibleError with the exception message (#49654)

* Don't just raise AnsibleError with the exception message. Fixes #49252

* Add changelog fragment

changelogs/fragments/vault-read-error.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- vault - Improve error messages encountered when reading vault files (https://github.com/ansible/ansible/issues/49252)

lib/ansible/parsing/vault/__init__.py

@@ -1030,7 +1030,10 @@ class VaultEditor:
                 with open(filename, "rb") as fh:
                     data = fh.read()
         except Exception as e:
-            raise AnsibleError(to_native(e))
+            msg = to_native(e)
+            if not msg:
+                msg = repr(e)
+            raise AnsibleError('Unable to read source file (%s): %s' % (to_native(filename), msg))
 
         return data