if textx is not installed, the check method prints the stacktrace of the Import Error

Bot 'sieve-expert': Validation of Sieve file failed with the following traceback: 'Traceback (most recent call last):\n File "/usr/lib/python3.13/site-packages/intelmq/bots/experts/sieve/expert.py", line 86,
in check\n metamodel = metamodel_from_file(grammarfile)\nTypeError: 'NoneType' object is not callable\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n File "/usr/lib/python3.13/site-packages/intelmq/bots/experts/sieve/expert.py", line 87, in check\n except TextXError as e:\n ^^^^^^^^^^\nNameError: name 'TextXError' is not defined\n'