To determine and vary the application of the zoning bylaws, ordinances or regulations in harmony with their general purpose and intent and with due consideration for conserving the public health, safety, convenience, welfare and property values solely with respect to a parcel of land where, owing to conditions especially affecting such parcel but not affecting generally the district in which it is situated, a literal enforcement of such bylaws, ordinances or regulations would result in exceptional difficulty or unusual hardship, so that substantial justice will be done and the public safety and welfare secured.