better error message

git-svn-id: http://xpra.org/svn/Xpra/trunk@3652 3bb7dfac-3a0b-4e04-842a-767bc560f471
setup: add cuda build rules (plus address-sanitizer build, should only be enabled for >= gcc 4.8)
csc_nvcuda: first attempt to first x264 to use nvcuda
add nvcuda C files
Fix incorrect printf reported by antoine
csc_nvcuda: add BGRX format string
csc_nvcuda: add get_flags_description() prototype
csc_nvcuda: Add BGRX src format to .pyx file
csc_nvcuda: fix typo