add option to fail instead of aborting when memory model is insufficient

[redianthus]

after #263

so that we don't say "nothing"

[redianthus]

@filipeom, out of curiosity, will there be such cases once the new memory model is working?

[filipeom]

I don't think so. I'm also trying to make the current memory model concretize instead of aborting.

[redianthus]

OK thus it's probably not worth adding the option for now if the plan is to remove it later.. :-)

[redianthus]

Should not be needed anymore. :)