Show meaningfull error messages (or error messages at all)

[mburri]

As an Admin, choose an Employee and set the Salary to 0 or empty value.
Save the employee.
Nothing happens, no feedback, nothing - except for the red borders around the salary input field - but the ahv-number input field is also highligthed, but saving without ahv-number is actually ok.

As a User, try to update the percentage for the next month payment during the lock period resolves with the following error message:

Actually - it should not be possible to change the value at all during the locking period.

edit: actually the frontend does lock up the form - because it keeps its own locking period! If the above error is shown if the date is changed in the backend. The problem here is that the information is kept redundant.