Errors not cleared on dimension under specific circumstances

[shimniok]

Reproduce: Enter invalid dimension into field 2+ times in a row, then attempt to correct the problem entering data into another field, and error indicator on first field not cleared.

Cause: using .add("error") on .getStyleClass() adds multiple error classes to the node.

Solution: check to make sure "error" is not already added before adding it.

[shimniok]

fixed