Following up on r1504501, gracefully exit in situations when the APR include
is missing instead of throwing out a cryptic stacktrace ("... local variable
'version_file_path' referenced before assignment").

* build/generator/gen_win_dependencies.py
  (_find_apr): Do not attempt to use the uninitialized 'version_file_path'
    variable in the error message, because we initialize it later.  Just
    stick to the same as in the _find_apr_util_and_expat(), i.e. use the
    hardcoded filename in the error message.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603901 13f79535-47bb-0310-9956-ffa450edef68