Fix/0141 regression snapshot freshness

[drewbanin]

fixes #1728

The full run-down:

• 0.14.1 auto-selects ephemeral models as a performance optimization
• the FreshnessRunner correctly handles non-source nodes, raising an exception (freshness runner: got a non-Source)
• this exception bubbles up to the queue handler
• dbt tries to mark nodes dependent on the ephemeral model as "skipped", if they exists
• the freshness runner doesn't support skipping, raises 'Freshness: nodes cannot be skipped!'
• the exception is swallowed by dbt and the process hangs

We actually do have a test for freshness tasks running alongside ephemeral models, but crucially, this hang only occurs if there is a model that _depends_on an ephemeral model in the project. I've update the test suite accordingly.

[beckjake]

Looks good to me, one very minor comment

[beckjake]

nit, but I would organize this like:

try:
    int(result.status)
except ValueError:
    logger.info("  Status: {}".format(result.status))
else:
    status = dbt.utils.pluralize(result.status, 'result')
    logger.info("  Got {}, expected 0.".format(status))

Otherwise uncaught ValueErrors in dbt.utils.pluralize will act as if your int failed.