Migrate to error return value instead of exceptions

[giann]

• We already have an available tag for errors in Value
• The parsing and type checking would not be impacted
• We could get rid of setjmp/longjmp
• Throwing and returning becomes the same thing
• try/catch would need to be removed
• Introduce try to be explicit when we propagate errors. This new keyword is required because we will have to implement a new bytecode that returns if an error is raised.

var another = try couldFail(); | return the error if fails
var again = couldFail() catch defaultValue; | default if fails

• New syntax to handle error based on its type to make up for the removed try/catch

var value = couldFail() catch {
   Unexpected -> defaultValue,
   else -> panic("you lost me")
}

• Additionally for this to be really useful we would need Blocks #105 so we could write

var value = couldFail() catch {
   Unexpected -> do {
       | ... some complex things

       break newValue;
   },
   else -> panic("you lost me")
}