Configuration sanity check

[dennybritz]

We should do a sanity check on the configuration upon loading. Instead of the application crashing in the middle of execution due to a configuration issue, we should immediately exit if we find an obvious mistake. Some things we can check for:

• Does the schema match the variables used in factor functions?
• Do all extractor UDF files exist?
• Do all extractor output relations exist?

There probably are more things that we can check for.

[rionda]

Zifei, this is an interesting idea if you would like to work on it.
Note that the active pipeline should be taken into account when doing the above checks.

[zifeishan]

Last time I made a typo in "calibration.holdout_query" and the system just did not holdout anything... We need this sanity check!