Increase robustness

[abraemer]

Right now this package is not very robust: Exceptions during writing a module leaves the startup.jl in a bad state, that does not work correctly and can't be parsed anymore. We should do:

• [ ] Improve parser to handle malformed startup.jl better. At the very least don't just error-out, but give the user a sensible error message.
• [ ] Make a safety copy of startup.jl prior to modification and roll back if errors occured.

[Alseidon]

A problem I found while using the package: add failed to add the code for a package, but still added the first line. So startup.jl now contains the starting line for one module, but nothing else.

This leads to errors by _parse_startup_file : as you do not check whether modules_starts and modules_ends have the same length, atline = module_ends[atmodule]+1 fails (because atmodule has a value of 1 and module_ends is empty.

I realize a fully robust parser would need to do more that check the two above lengths are equal, but this would improve a startup.jl state which the package itself generated.