Fix sign of feedforward term in Task::computeTaskExponentialDecrease.

  \dot{e} &=& J\dot{q} + \frac{\partial e}{\partial t}\\
  \dot{q} &=& J^{+} (\dot{e}^{*} - \frac{\partial e}{\partial t})\\
          &=& J^{+} (-\lambda e - \frac{\partial e}{\partial t})