Adds slightly richer errors raised during the local merge.

The following properties may now hold useful information on a captured
sqlalchemy.ext.IntegrityError exception during the local merge, which
is called by the pull procedure:

- conflicting_class: the class of the tracked object that triggered
  the error

- conflicting_pk: the primary key value of the tracked object

- conflicting_column: a string with the name of the column that's
  involved in the error

- conflicting_value: the value that caused the conflict

Their values will be None if the library can't find values to give
them.