Use nonzero status code to signal abnormal termination

We currently return with status code 0 when an IOException occurs.
The plugin manager should in any case return a nonzero status if
the operation was not successful. Now the PluginManager uses the
following reponse codes based on 'sysexists.sh':
 * '0' on success
 * '64' command line usage error
 * '70' internal software error
 * '74' input/output

Closes #3463