gui/main: Fix migration error details construction
gui/main: Fix migration error details construction

  Since we can run multiple migrations and get back multiple errors, we
  want to list all of them when notifying the user.
  However, when building that list we were not assigning the result of
  the concatenation of each error message to the details variable so the
  list would always be empty.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop