include inputfile in errors

[st--]

When using Literate in parallel processes, it would be very helpful for debugging if the error messages would actually include the name of the original input file - as discussed in JuliaGaussianProcesses/KernelFunctions.jl#303 (comment). This PR includes a draft version of how this can be achieved. Happy to find better ways of doing it:)

[devmotion]

include_string allows to specify a filename as additional argument. In this way the original error message that is rethrown by Literate would contain the filename automatically I assume.

Alternatively but a bit more involved, maybe one could also keep track of the line numbers and rethrow a LoadError and forward the filename and linenumber to it.

[st--]

Keeping track of line numbers would be difficult because the preprocess function could arbitrarily change it..

[fredrikekre]

Thanks, I think this is good enough. Line location would be nice, but since you see the code block and now know the file it should be pretty easy to find the location.