Fix MAGN-919 Errors disappear if user clicks on the node

Background
==========
This is meant to fix the following defect (the behavioral changes are validated by Ian):

[MAGN-919 Error disappear if user click on the node and then on canvas](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-919)

It solves a few issues:

 * **ValidateConnections** causes erroneous nodes to be valid again
 * **FormulaNode** doesn't turn red when it has error until "Run"
 * Error tool-tip text gets reset after a node is deselected

Unit testing
============
* All existing test cases are passing as expected
* Test case **ValidateConnectionsDoesNotClearError** added to verify the new behavior