No input error message if tax rate is missing

[dellermann]

If the user forgets to enter a tax rate in invoices, quotes etc. the form will not be saved and an error is displayed below the item list. However, no error message is displayed above the form.

[dellermann]

Maybe a setting should be implemented where the user may define a default tax rate and a default tax rate depending on the organization. That default tax rate should be used if the user doesn't enter one.