Fix #59

[sharpaper]

Return null if the .hdt file given in the config file doesn't exist.

[mielvds]

Thanks for the PR! Can you explain why returning null is more helpful than an exception?

[mielvds]

After seeing #59, I'm ok with this PR? However, could you adjust the catch clause with a more accurate message?

[sharpaper]

Thanks for the PR! Can you explain why returning null is more helpful than an exception?

Thread.

The point is that it would be nice to have a flag that allows me to skip those graphs that are defined in the config file but miss the .hdt source, rather than throwing an error. I'm not sure though, what is the best way to handle this... If it should be handled by Fuseki or by hdt-java. I'm closing this PR for now...