Show a warning in the log if a library is missing

I've run into a situation where some of the required libraries were missing in the libraries folder. (This might have been caused by a connection issue on my side.)
Since I did not find a warning/error message in the log telling me a file is missing it took a while to find the cause.
I suggest to at least add a warning to the log (as done in the attached commit). Since in some cases Minecraft might work even though a library is missing I think the launch should not be instantly aborted when a file is missing.