Give more context in errors about running the WC DB work queue.  Instead
of just giving the low-level error such as "file does not exist", also give a
"failed to run the work queue" error with WQ context info.

* subversion/libsvn_wc/workqueue.c
  (dispatch_work_item): Remove some detail from an error message here.
  (svn_wc__wq_run): Wrap any error in running a work item (including the
    error above) in an error message that contains the work item id and skel.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1434013 13f79535-47bb-0310-9956-ffa450edef68