[PATCH] improve error output with missing apr binary directory

Hi,

when generating the project files on Windows and the apr bin-directory
being missing the python gen-make.py call currently produces the
following cryptic error message:

Traceback (most recent call last):
  File "gen-make.py", line 326, in <module>
    main(conf, gentype, skip_depends=skip, other_options=rest.list)
  File "gen-make.py", line 62, in main
    generator = gen_module.Generator(fname, verfname, other_options)
  File "build\generator\gen_vcnet_vcproj.py", line 36, in __init__
    'vcnet-vcproj')
  File "build\generator\gen_win.py", line 83, in __init__
    self.find_libraries(True)
  File "build\generator\gen_win_dependencies.py", line 299, in
find_libraries
    self._find_apr()
  File "build\generator\gen_win_dependencies.py", line 408, in _find_apr
    bin_files = os.listdir(dll_dir)
WindowsError: [Error 3] The system cannot find the path specified:
'[...]\\dependencies\\httpd\\srclib\\apr\\bin/*.*'

The attached patch adds a check to produce a better readable error instead:

ERROR: no apr bin folder detected.
Make sure to have apr built prior to generating the SVN project.

Regards,
Stefan