Inconsistent Input Validation: stopifnot() vs abort() in helper functions

[utkarshpawade]

Description

Several parts of the codebase use a mix of stopifnot() and rlang::abort() for input validation. While abort() provides clear, user-friendly error messages, stopifnot() produces opaque messages that are harder for users to interpret.

This leads to inconsistent error handling and a less friendly user experience when validation fails.

Example

In helpers-ppc.R:

# Line 70
stopifnot(is.matrix(predictions), is.numeric(predictions))

# Line 85
abort("NAs not allowed in predictions.")