two small fixes to gaia_object.py

[JoanneBogart]

Fix two things affecting gaia objects:

1. Interface for load_collection changed a while back to include a new optional argument, exposure, but GaiaCollection.load_collection was not updated at the time. It's done in this PR
2. A while ago a config value id_prefix in the section about Gaia stars. If it was omitted from the config and someone wanted to include Gaia stars in their search, skyCatalogs would crash. The fix here is to use a default value if the key is omitted from the config.

[jchiang87]

lgtm