migration : fix migrateAccount()

- bug : if readArchive() line 1410 throw an exception, migrateAccount()
will return without emiting any signal.

- you can reproduce it by passing a wrong password to readArchive().

- this patch fix the try catch by adding a signal before the return.

- fix a missing else line 1421. Whitout it, whenever if
updateCertificates return true, a Migration::State::INVALID signal is
emited. Wich also leads when updateCertificates is true to emit two
opposite signals.

Change-Id: Id3bdbe6ef852095793af7802c75559d033b9f890
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>