`AttributeError` not handled in `basesections.CompositeSystem`

Quickly fixable

When adding a PureSubstanceComponent to CompositeSystem.components, if component.pure_substance.molecular_formula is missing, the code encounters an attribute error.

The error can be re-created with:

from nomad.client import normalize_all, parse
from nomad.datamodel.metainfo.basesections import PureSubstanceComponent

sample_archive = parse('test.archive.yaml')[0]
sample_archive.data.components.append(PureSubstanceComponent())
normalize_all(sample_archive)

where test.archive.yaml has following content:

data:
  m_def: nomad.datamodel.metainfo.eln.ELNSample

The code block where the error comes from is already inside a try and checks for ValueError. Extending this to include AttributeError can be a quick fix.