Improve error handling when sessions expire

[antgonza]

When an admin is in one of their pages and the session expires the GUI shows a DataTables error:

ERROR:
'NoneType' object has no attribute 'email'
TRACE:
Traceback (most recent call last):

File "/home/qiita/miniconda3/envs/qiita/lib/python3.9/site-packages/tornado/web.py", line 1592, in _execute
result = yield result

File "/home/qiita/miniconda3/envs/qiita/lib/python3.9/site-packages/tornado/gen.py", line 1133, in run
value = future.result()

File "/home/qiita/miniconda3/envs/qiita/lib/python3.9/site-packages/tornado/gen.py", line 307, in wrapper
result = func(*args, **kwargs)

File "/home/qiita/miniconda3/envs/qiita/lib/python3.9/types.py", line 278, in wrapped
coro = func(*args, **kwargs)

File "/home/qiita/qiita/qiita_core/util.py", line 83, in wrapper
return func(*args, **kwargs)

File "/home/qiita/qiita/qiita_pet/handlers/admin_processing_job.py", line 48, in get
self._check_access()

File "/home/qiita/qiita/qiita_pet/handlers/admin_processing_job.py", line 31, in _check_access
self.current_user.email)

AttributeError: 'NoneType' object has no attribute 'email'