Avoiding UTF-8 failures

Hey guys,

Love your work, we used it for 5+ years now.

We're having an issue sometimes with some UTF-8 failure when getting metadata. Basically when sometimes something can't be converted to UTF-8 we get "Failure: Utils.utf_8.next" instead of all the infos.

I made this dirty fix (I don't know OCaml, and no offense but I found it absolutely incomprehensible and incoherent). The fix return key=ERROR when a failure happens, instead of blocking all the metadata.

If you can do better, and that wouldn't be difficult, please do it ^^

Thanks.