Implemented error handling for XBlock instances
Merge pull request #240 from edx/lmorales/xblock-error-handling

Implemented error handling for XBlock instances
Provide ability to set field to a unique string by default.

This patch makes it possible to set a field's default value to
a unique string that appears random, but is deterministically
calculated for a field in the given scope:

    my_field = xblock.fields.String(default=xblock.fields.UNIQUE_ID, ...)