AEB43 import raises traceback instead of validation error when file is invalid
When a user imports an invalid AEB43 file, the system raises a traceback instead of showing a user-friendly validation error:
```
Traceback (most recent call last):
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/wsgi.py", line 98, in dispatch_request
return endpoint(request, **request.view_args)
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/protocols/dispatcher.py", line 44, in rpc
return methods.get(request.rpc_method, _dispatch)(
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/wsgi.py", line 78, in wrapper
return func(request, *args, **kwargs)
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/protocols/wrappers.py", line 206, in wrapper
result = func(request, pool, *args, **kwargs)
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/protocols/dispatcher.py", line 216, in _dispatch
result = rpc.result(meth(*c_args, **c_kwargs))
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/wizard/wizard.py", line 347, in execute
result = wizard._execute(state_name)
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/wizard/wizard.py", line 382, in _execute
do_result = do(action)
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/modules/account_statement/statement.py", line 1273, in do_import_
statements = list(getattr(self, 'parse_%s' % self.start.file_format)())
File "/home/resteve/projectes/nandev/nan76/trytond/trytond/modules/account_statement_aeb43/statement.py", line 27, in parse_aeb43
aeb43 = AEB43(file_)
File "/home/resteve/projectes/nandev/lib/python3.13/site-packages/aeb43/__init__.py", line 28, in __init__
self._parse(name)
File "/home/resteve/projectes/nandev/lib/python3.13/site-packages/aeb43/__init__.py", line 45, in _parse
account.transactions.append(transaction)
AttributeError: 'NoneType' object has no attribute 'transactions'
```
The error message should be user-friendly and should not expose internal traceback details.
issue