Show an error for path data that contains errors

[romannurik]

In the property inspector, since you can manually enter path data, show an error when there's a problem parsing the path data

[alexjlockwood]

Showing an error when attempting to animate between two incompatible paths would be nice too.

[romannurik]

Currently entering an invalid path just reverts back to the previous value. An error state of some kind would be nice.