* include/wget/wget.h: Add wget_buffer.error
* libwget/buffer.c: Amend docs,
  change return type of wget_buffer_init(),
  amend wget_buffer_alloc(),
  set buf.error in _buffer_realloc()
* libwget/printf.c: Check buf.error, return (size_t)-1
* unit-tests/test.c: Amend tests appropriately