Clarify an error message.

Foudn by: Marshall Schor

* subversion/libsvn_wc/props.c
  (validate_eol_prop_against_file): 
    Have an error message spell out the attempted operation.  It's not always
    clear from context (e.g., with autoprops).


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461328 13f79535-47bb-0310-9956-ffa450edef68