[IMP] project: improve the user/validation errors

The purpose of the PR is, User/validation errors are sometimes not
correct in English and/or quite obscure and don't help the user
understand/solve the issue. so some copywriting should help make
the experience better.

task-2557805

closes odoo/odoo#80107

Related: odoo/enterprise#22402
Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>

addons/project/models/project.py

@@ -336,7 +336,7 @@ class Project(models.Model):
     is_milestone_exceeded = fields.Boolean(compute="_compute_is_milestone_exceeded", search='_search_is_milestone_exceeded')
 
     _sql_constraints = [
-        ('project_date_greater', 'check(date >= date_start)', 'Error! Project start date must be before project end date.')
+        ('project_date_greater', 'check(date >= date_start)', "The project's start date must be before its end date.")
     ]
 
     @api.depends('partner_id.email')
@@ -2291,7 +2291,7 @@ class ProjectTags(models.Model):
     task_ids = fields.Many2many('project.task', string='Tasks')
 
     _sql_constraints = [
-        ('name_uniq', 'unique (name)', "Tag name already exists!"),
+        ('name_uniq', 'unique (name)', "A tag with the same name already exists."),
     ]
 
     @api.model