--render-from: Handle exceptions if previous functionalities have not been yet rendered

[tjazerzen]

Let's say you have module hello_world_python.plain, which hasn't been rendered at all, and you run codeplain hello_world_python.plain --render-from 2 (assuming there are 2 FSs).

• Before: All the modules listed under "requires" would be rendered, and only after that we could crash because functional spec 1 for hello_world_python.plain hasn't been implemented yet.
• After: We detect that functional spec 1 is missing and we provide informative error message before doing any generation beforehand (refer to the photo below).

IMHO this is better UX.

[kaja-s]

I think we should align on the messaging and communication of functional specs.

I see "functional specs" as a section or heading for "functionalities". Meaning:

*** functional specs ***

• functionality_1
• functionality_2
• functionality_3
  ...

If we agree on this separation, then the error message would be as follows:

Error rendering ***plain code: Cannot start rendering from functionality 2 for module hello_world_python because the implementation of the previous functionality (1) hasn't been completed yet.
To fix this, please render the missing functionality (1) first by running:
codeplain hello_world_python.plain --render-from-1