data source layer should have useful exceptions

[marienz]

Currently an attempt to open a local_source with a bogus path lets the IOError reach calling code. There should be some data_source specific exceptions here so calling code can deal with various kinds of sources sensibly (checking for ENOENT probably does not make sense for networked data sources). Todo:

• come up with a good set of exceptions
• make the existing sources raise them and not let IOError/OSError through
• remove the ugly hack I just added to repo_objs.py

[ferringb]

imo, local_source consumers should *know* the source is available; in the specific case of metadata.xml parsing, I was being lazy and attempting to avoid a stat'ing instead of honoring that implicit req of local_source.

Meanwhile, backed out the ugly hack and slipped in LocalMetadataXml? that handles nonexisting metadata.xml at the time of forced parsing.

[ferringb]

@marienz: thoughts/comments on this? Honestly haven't really poked at it much in light of comment #2 :)