Current bounce message is not very user friendly. Purpose of this commit is to improve it, by improving wording and overall phrasing used in it. Form view is improved so that bounce message takes all available width in form view. Some tests are added, notably to detect pseudo-void content from editor. Task ID-2532529 PR odoo/odoo#71793 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>