[notification] avoid leaking cnxsets (closes #3243810)
When sending notifications, we get each recipient as either an email
address or a CWUser. In the latter case, we create a temporary session
for that user and use it to send the mail. However, if we later decided
to not send the mail after all, we'd leak the session and its cnxset.
Add a try block inside the loop to make sure the temporary sessions are
closed properly.
dn: dc=cubicweb,dc=test
structuralObjectClass: organization
objectClass: dcObject
objectClass: organization
o: cubicweb
dc: cubicweb
dn: ou=People,dc=cubicweb,dc=test
objectClass: organizationalUnit
ou: People
structuralObjectClass: organizationalUnit
dn: ou=Group,dc=cubicweb,dc=test
objectClass: organizationalUnit
ou: Group
dn: cn=logilab,ou=Group,dc=cubicweb,dc=test
gidNumber: 2000
objectClass: posixGroup
objectClass: top
cn: logilab
memberUid: adim
dn: cn=dir,ou=Group,dc=cubicweb,dc=test
gidNumber: 2002
objectClass: posixGroup
objectClass: top
cn: dir
memberUid: adim
memberUid: syt
dn: uid=syt,ou=People,dc=cubicweb,dc=test
loginShell: /bin/bash
objectClass: OpenLDAPperson
objectClass: posixAccount
objectClass: top
objectClass: shadowAccount
cn: Sylvain Thenault
sn: Thenault
shadowMax: 99999
gidNumber: 1004
uid: syt
homeDirectory: /home/syt
shadowFlag: 134538764
uidNumber: 1004
givenName: Sylvain
telephoneNumber: 106
displayName: sthenault
gecos: Sylvain Thenault
mail: sylvain.thenault@logilab.fr
mail: syt@logilab.fr
userPassword: syt
dn: uid=adim,ou=People,dc=cubicweb,dc=test
loginShell: /bin/bash
objectClass: OpenLDAPperson
objectClass: posixAccount
objectClass: top
objectClass: shadowAccount
cn: Adrien Di Mascio
sn: Di Mascio
shadowMax: 99999
gidNumber: 1006
uid: adim
homeDirectory: /home/adim
uidNumber: 1006
givenName: Adrien
telephoneNumber: 109
displayName: adimascio
gecos: Adrien Di Mascio
mail: adim@logilab.fr
mail: adrien.dimascio@logilab.fr
userPassword: adim