$page->save($fieldname) error reporting / exception handling does not work with FieldtypeTextUnique

[mtwebit]

I'm trying to store values in a FieldtypeTextUnique field using the API.
Unfortunately, the following code reports no error / exception when a value is not stored because it is not unique in the DB.

try {
  if (!$page->save($fieldname)) {
    ... report the error ...
  }
} catch (\Exception $e) {
  .. report the exception ...
}

I was expecting at least an exception because of the UNIQUE SQL constraint.
Any idea how to catch this error in a module that modifies page fields?
I also tried the validate the saved value but things got complicated for certain field types (e.g. datetime, WireArrays).
Thanks!