Flag to throw fatal error when missing file in interactive mode.

[tpersson]

A missing file caused a problem in the upload of an optics. This was not spotted since JMAD runs in interactive mode and calling a missing file only triggers a warning. This PR adds a flag to make this fatal also in interactive mode. The default behaviour is as in the past.

[rdemaria]

Any reason not to make the flag also valid for non-interactive mode?

[tpersson]

For the non-interactive mode, the behaviour is always to exit which I think is fine. If one would change the default behaviour so that the interactive exits by default then having the flag effecting both would make sense but I didn't want to change the default behaviour based on my experience with all the complaints we receive when we do...

[rdemaria]

ok!

[rdemaria]

In general for interactive work, I always use no_fatal_stop because it is common to make mistakes or try things out without being afraid of restarting from scratch.

[tpersson]

Yeah, indeed this is basically doing the opposite. It is really for the cases where it is not really interactive. It is for things such as JMAD and also the beta-beat.src where we still have a parser that opens a MAD-X in a stream.