[!124] Fix warning: variable 'errorflag' set but not used

Merge branch 'fix_warning_set_but_unused_errorflag' into 'master'

ref:staging/dune-uggrid

    I added the attribute "DUNE_UNUSED" to the variable "errorflag" to avoid the
     warning: "variable 'errorflag' set but not used".

See merge request [!124]

  [!124]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/124