Return send mail status in invitation creation api

[touilleMan]

When mail cannot be sent to the mailing server we get an error code that is correctly only logged:

 May 28 21:26:22 parsec-cloud-prod app/web.1 2021-05-28 19:26:22 [warning  ] SMTP error                     [parsec.backend.invite] to_addr=xxx@yyy
May 28 21:26:22 parsec-cloud-prod app/web.1     raise SMTPDataError(code, resp)
May 28 21:26:22 parsec-cloud-prod app/web.1 smtplib.SMTPDataError: (550, b'5.7.1  Reject for policy reason RULE3_2. See http://postmaster.gandi.net')
May 28 21:32:04 parsec-cloud-prod app/web.1 2021-05-28 19:32:03 [warning  ] SMTP error                     [parsec.backend.invite] to_addr=xxx@yyy
May 28 21:32:04 parsec-cloud-prod app/web.1     raise SMTPDataError(code, resp)
May 28 21:32:04 parsec-cloud-prod app/web.1 smtplib.SMTPDataError: (550, b'5.7.1  Reject for policy reason RULE3_2. See http://postmaster.gandi.net')
May 28 21:41:19 parsec-cloud-prod app/web.1 2021-05-28 19:41:19 [warning  ] SMTP error                     [parsec.backend.invite] to_addr=xxx@yyy
May 28 21:41:19 parsec-cloud-prod app/web.1     raise SMTPDataError(code, resp)
May 28 21:41:19 parsec-cloud-prod app/web.1 smtplib.SMTPDataError: (550, b'5.7.1  Reject for policy reason RULE3_2. See http://postmaster.gandi.net')

It would be better to provide this information to the client so it can inform the user the mail couldn't be send, then display the invitation link and suggest to provide it to the invitee directly.