improve error message when plugin not found

If a plugin currently is not found, the error message looks like this:
```
> actual_score = score(model_identifier=model_identifier, benchmark_identifier=benchmark_identifier)

test_integration.py:17:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../brain-score_language/brainscore_language/__init__.py:79: in score
return wrap_score(__file__,
../../brain-score_core/brainscore_core/plugin_management/conda_score.py:87: in wrap_score
result = score_function(model_identifier, benchmark_identifier)
../../brain-score_language/brainscore_language/__init__.py:54: in _run_score
model: ArtificialSubject = load_model(model_identifier)
../../brain-score_language/brainscore_language/__init__.py:42: in load_model
import_plugin('brainscore_language', 'models', identifier)
../../brain-score_core/brainscore_core/plugin_management/import_plugin.py:79: in import_plugin
importer = ImportPlugin(library_root, plugin_type, identifier)
../../brain-score_core/brainscore_core/plugin_management/import_plugin.py:19: in __init__
self.plugin_dirname = self.locate_plugin()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self =

def locate_plugin(self) -> str:
"""
Searches all `plugin_type` __init.py__ files for the plugin denoted with `identifier`.
If a match is found of format {plugin_type}_registry[{identifier}],
returns name of directory where __init.py__ is located
"""
plugins = [d.name for d in self.plugins_dir.iterdir() if d.is_dir()]

specified_plugin_dirname = None
plugin_registrations_count = 0
for plugin_dirname in plugins:
if plugin_dirname.startswith('.') or plugin_dirname.startswith('_'): # ignore e.g. __pycache__
continue
plugin_dirpath = self.plugins_dir / plugin_dirname
init_file = plugin_dirpath / "__init__.py"
with open(init_file) as f:
registry_name = self.plugin_type.strip(
's') + '_registry' # remove plural and determine variable name, e.g. "models" -> "model_registry"
plugin_registrations = [line for line in f if f"{registry_name}['{self.identifier}']"
in line.replace('\"', '\'')]
if len(plugin_registrations) > 0:
specified_plugin_dirname = plugin_dirname
plugin_registrations_count += 1

> assert plugin_registrations_count > 0, f"No registrations found for {self.identifier}"
E AssertionError: No registrations found for pixels

../../brain-score_core/brainscore_core/plugin_management/import_plugin.py:45: AssertionError
```

I can see this quickly being overwhelming to the average user, e.g. "registrations" probably does not make immediate sense.