Fix resetting of the error state for FileEditor

[greschd]

Reset the 'error' state of the file editor when its value changes. Previously, the state would remain
as error, which means the value cannot be set.

Created as draft because this only implements it for wx, whereas qt has the same issue.

The code block of the _reset_error function

if self._error is not None:
    self._error = None
    self.ui.errors -= 1 
    self.set_error_state(False)

is then present in four places: text_editor and file_editor for both qt and wx. Since I'm not really familiar with the overall architecture of the code: What's the best place to put this? Or should I keep it separate?