UX: Add explicit feedback when file has no errors

[pdelboca]

This issues is a follow up of #1076

Only disabling the Errors Button when there are no errors might cause some confusion since it is not quite clear why the button is disabled. Even when the status bar displays a message, the button's UX could be improved. Something similar happened with the Source View, where the disabled button was confusing: #979

We should implement the same UX for the errors button:

• If no errors are found, the button should be enabled and a "The file has been processed and no errors has been found," message should be displayed.
• In addition, we can add a green zero number to the button itself.