[FIX] mail: show correctly qweb error

Before this commit it is impossible to find the qweb error when you preview an email template.

closes odoo/odoo#139668

X-original-commit: 1f373de5f218a1f43015f4f80e60d1da0188d6b5
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/models/mail_render_mixin.py

@@ -5,6 +5,7 @@ import babel
 import copy
 import logging
 import re
+import traceback
 
 from lxml import html
 from markupsafe import Markup
@@ -325,8 +326,9 @@ class MailRenderMixin(models.AbstractModel):
                     ) from e
                 _logger.info("Failed to render template: %s", template_src, exc_info=True)
                 raise UserError(
-                    _("Failed to render QWeb template: %(template_src)s)",
-                      template_src=template_src)
+                    _("Failed to render QWeb template: %(template_src)s\n\n%(template_traceback)s)",
+                      template_src=template_src,
+                      template_traceback=traceback.format_exc())
                     ) from e
             results[record.id] = render_result
 

addons/mail/wizard/mail_template_preview_views.xml

@@ -11,7 +11,7 @@
                             <h3>Preview of <field name="mail_template_id" readonly="1" nolabel="1" options="{'no_open': True}"/></h3>
                         </div>
                         <div class="alert alert-danger" role="alert" invisible="not error_msg">
-                            <field name="error_msg" />
+                            <field name="error_msg" widget="text"/>
                         </div>
                         <field name="no_record" invisible="1"/>