LogProcessor: more error-handling modes besides "skip" and "raise"

[Antar1011]

Right now the only two error-handling options are:

'''
                    * "raise" : raise an exception if an error is encountered.
                    * "skip" : silently skip problematic logs
'''

This means that either one bad log ruins the whole batch or that bad logs don't get reported.

"Robust error handling" is currently targeted for the 0.7 release, but in order to get through "a whole month of logs" this will need to be dealt with sooner (0.4 release).

For now, I propose just a "warn" strategy that prints the error message but keeps chugging.

[Antar1011]

If I want to punt this to 0.4, the workaround for now is to do this manually by processing logs one-at-a-time (using glob to get the list of logs).

[Antar1011]

I think this time around I'm going to do "warn" as well.