Warnings (locked, moved to epic #398)

[OAGr]

Right now Squiggle-lang occasionally returns errors. It should also return warnings.

For example:

• When toPointSet or toSampleSet is called on a variable, and accuracy is lost.
• If a variable undergoes log() and some of it is removed because it will help log() succeed.
• If a key variable is nonNormalized.
• If a function takes an unusually long amount of time.

This would require us to have something more advanced than our current result<> type.

[quinn-dougherty]

Still the same philosophy as result tho, right, but success is a tuple.

type ourResult<('a, 'b), 'c> = 
  | Ok((x, y))
  | Error(z) 

'b = string 90% of the time unless we want to leverage variants for warnings.

[OAGr]

(I just spoke about this with @quinn-dougherty . We discussed making warnings an array, within the result tuple, or similar. We'd want to handle multiple warnings.

[quinn-dougherty]

Moved to #398 (the epic)