djvu.c: return internal djvulibre error (#638)
djvu.c: return internal djvulibre error (#638)

All of the `GURL` stuff in djvulibre is absurdly complicated and this does nothing to help, but it does give nicer error messages in case of failure.

References https://github.com/koreader/koreader/issues/542

```
04/06/18-10:20:48 WARN  cannot open document /mnt/onboard/.books/koreader_test/Михайлов ТФКП.djvu frontend/document/djvudocument.lua:42: ddjvu: ** Unrecognized DjVu Message:
	** Message name:  ByteStream.open_fail
	   Parameter: %D0%9C%D0%B8%D1%85%D0%B0%D0%B9%D0%BB%D0%BE%D0%B2%20%D0%A2%D0%A4%D0%9A%D0%9F.djvu
	   Parameter: No such file or directory
ddjvu: 'ByteStream.cpp:704'
```
NiLuJe/koreader-base,houqp/koreader-base,houqp/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,koreader/koreader-base,Frenzie/koreader-base,koreader/koreader-base,koreader/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base