Make the ra_svn detect connection failures (such as a terminated client)
more quickly by actually passing detected errors down the stack.

This also fixes the associated error leak(s).

* subversion/libsvn_ra_svn/marshal.c
  (svn_ra_svn__write_string,
   svn_ra_svn__write_cstring): Don't swallow the svn_ra_svn__write_ncstring
                               return values.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590984 13f79535-47bb-0310-9956-ffa450edef68