populate metadata error handling

[will-moore]

E.g. see errors reported at https://forum.image.sc/t/how-to-use-cellprofiler-per-image-results-from-plate-analyses-as-source-for-omero-parade/59894/19

The actual error ValueError: Empty Double or Long value. Use --allow_nan to convert to NaN is buried among lots of others.
including:

Traceback (most recent call last):
  File "./script", line 124, in populate_metadata
    ctx.parse_from_handle(data)
AttributeError: 'ParsingContext' object has no attribute 'parse_from_handle'

Probably the metadata plugin needs to do better too.