Add more hits to the error "Error msg: bad conversion"

[Marco Marchitto]

Hello,

when I try to parse an empty yaml file I got the error "Error msg: bad conversion".

Could this error message be improved a bit?

For example, if I try to get a key that is not in the yaml file, the error message could say that the key is not in the file. Also, specifying that the error comes from yaml-cpp (e.g "Error msg: bad conversion when parsing <file_name>.yaml") can be very helpful because you can easily understand from where the issue comes from. In particular when using yaml-cpp inside a big software architecture.

Thanks!

[chenxu]

same requirement.