def handle_missing_dependency(entry_point: pkg_resources.EntryPoint,
err: ModuleNotFoundError) -> str:
"""Return a suitable error message for a missing optional dependency.
Args:
entry_point: The entry point that was attempted to load but caused
a ModuleNotFoundError.
err: The ModuleNotFoundError that was caught.
Re-raises the given ModuleNotFoundError if it is unexpected.
"""
try:
# Check for missing optional dependencies
entry_point.require()
except pkg_resources.DistributionNotFound as exc:
# Confirmed missing optional dependencies
return f"cylc {entry_point.name}: {exc}"
else:
# Error not due to missing optional dependencies; this is unexpected
raise err
