add EESSIError exception to avoid printing traceback

[smoors]

changes this:

ERROR: run session stopped: reframe fatal error: Invalid thread_binding value 'nonexisting'. Valid values: 'true', 'compact', or 'false'.
ERROR: Traceback (most recent call last):
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/frontend/cli.py", line 1354, in main
    checks_found = loader.load_all(force=True)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/logging.py", line 1121, in _fn
    return fn(*args, **kwargs)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/frontend/loader.py", line 244, in load_all
    checks += self.load_from_dir(d, self._recurse, force)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/frontend/loader.py", line 223, in load_from_dir
    checks += self.load_from_file(entry.path, force)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/frontend/loader.py", line 195, in load_from_file
    return self.load_from_module(util.import_module_from_file(
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/frontend/loader.py", line 156, in load_from_module
    candidate_tests = registry.instantiate_all(reset_sysenv,
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/logging.py", line 1121, in _fn
    return fn(*args, **kwargs)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/decorators.py", line 105, in instantiate_all
    leaf_tests.append(test(*args, **kwargs))
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/meta.py", line 497, in __call__
    obj.__init__(*args, **kwargs)
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/hooks.py", line 111, in _fn
    getattr(obj, h.__name__)()
  File "/vscmnt/brussel_pixiu_data/_data_brussel/100/vsc10009/reframe/eessitestsuite/venv-reframe/lib64/python3.9/site-packages/reframe/core/hooks.py", line 38, in _fn
    func(*args, **kwargs)
  File "/data/brussel/100/vsc10009/reframe/eessitestsuite/test-suite/eessi/testsuite/eessi_mixin.py", line 159, in EESSI_mixin_run_after_init
    raise ReframeFatalError(err_msg)
reframe.core.exceptions.ReframeFatalError: Invalid thread_binding value 'nonexisting'. Valid values: 'true', 'compact', or 'false'.

to this:

ERROR: run session stopped: eessierror: Invalid thread_binding value 'nonexisting'. Valid values: 'true', 'compact', or 'false'.
ERROR: eessi.testsuite.utils.EESSIError: Invalid thread_binding value 'nonexisting'. Valid values: 'true', 'compact', or 'false'.