message: try/catch block around callback
  allows it to recover from errors that are in the callback
  fixes
diff: @@ -193,11 +193,27 @@ define(["Tone/core/Tone", "Tone/signal/TimelineSignal", "Tone/core/TimelineState
  if (this.frequency){
  this._nextTick += 1 / this.frequency.getValueAtTime(this._nextTick);
  if (currentState === Tone.State.Started){
  - this.callback(tickTime);
  + var error = this._tryCallback(tickTime);
  this.ticks++;
  + if (error){
  + throw new Error(error);
  }
  }
  }
  + }
  + };
  +
  + /**
  + * Invoke the callback with a try/catch block
  + * @param {Time} time The time to invoke the callback
  + * @private
  + */
  + Tone.Clock.prototype._tryCallback = function(time){
  + try {
  + this.callback(time);
  + } catch(e){
  + return e;
  + }
  };
  /**
  