[#86,bugfix][s]: log errors rather than crash when provided bad dates - fixes #86.

Tested with this spreadsheet: https://docs.google.com/a/okfn.org/spreadsheet/ccc?key=0AqR8dXc6Ji4JdERlNXQ3ekttQk5USmFRaTFMYUNJTkE&usp=drive_web#gid=0