When a merge aborts due to conflicts, don't raise an error at such a low
level as we were doing, but in a single place at a slightly higher level.
A small step torwards being able to keep going if the user resolves all the
conflicts, for issue #4316 "Merge errors out after resolving conflicts".

* subversion/libsvn_client/merge.c
  (do_file_merge,
   do_mergeinfo_unaware_dir_merge,
   do_mergeinfo_aware_dir_merge,
   do_directory_merge): If we raise a conflict, return the revision range
    during which that occurred instead of an error describing it.
  (do_merge): Raise the error here instead -- consistently.

* subversion/tests/cmdline/merge_tests.py
  (conflict_aborted_mergeinfo_described): Test directory merges as well as
    single-file merges.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1444453 13f79535-47bb-0310-9956-ffa450edef68