Added runtime errors when reading files.

[OGaboriault]

Description of the problem

• The code doesn't crash when it tries to open a file that doesn't exist in the current directory. In my opinion, this is not a feature...
• i.e., tried to insert particles with the file insertion and the simulation ran for many hours with no particles in it because it didn't crash.

Comments

• I have put Lucka as a reviewer, because it may be a "feature" for sharp.

[blaisb]

I don't think that's a feature anywhere. It's just a lack of sanity checking.

[Luckabarbeau]

Nothing to add here ! seems good to me.