Fix ``_ErrorLog`` not iterable

[nsoranzo]

Fix the following mypy error:

lib/galaxy/tool_util/linters/xsd.py:28: error: "_ErrorLog" has no attribute
"__iter__" (not iterable)  [attr-defined]
                for error in xsd.error_log:
                             ^~~~~~~~~~~~~

xref: https://github.com/galaxyproject/galaxy/pull/17081/files#r1490662640