tmpfiles: fix obscure leak in error path

The leak was because of the single return in midst of all
'goto finish'es. Using automatic cleanup simplifies things.
journalctl: reword error message

Sentence seemed to suggest that all three conditions must be true.
activate: do not leak envp on error
units/user: default.target must be isolatable

...
Activating default unit: default.target
Default target could not be isolated, starting instead: Operation refused, unit may not be isolated.
Partially revert e62d8c394474

The ~80 chars per line part wasn't well received.