Detect non-finite `log_lik` (`Inf` / `NaN`) and explicitely inform the user

Summary

If the pointwise log-likelihood matrix contains +Inf or NaN, loo::loo() currently completes without a clear error/warning and can return NA for elpd_loo.

Proposal

Before (or while) computing PSIS-LOO, check the log_lik matrix for non-finite values and message this clearly to the user.

Reproducible example

library(loo)

set.seed(1)
ll <- matrix(rnorm(400 * 20, -1, 0.5), 400, 20)
ll[1:5, 1] <- Inf
loo(ll)$estimates  # elpd_loo is NA; no clear message that log_lik had +Inf
#          Estimate SE
# elpd_loo       NA NA
# p_loo          NA NA
# looic          NA NA

Context

Seen with truncated-normal log_lik from brms, where the truncation normalizer can underflow:
https://discourse.mc-stan.org/t/difficulty-adding-loo-criterion-to-longitudinal-truncated-normal-model/41447/12

Corresponding brms Issue: https://github.com/paul-buerkner/brms/issues/1899

Start at the loo::loo() entry point and trace where the log_lik matrix is validated or passed into PSIS-LOO. Use the reproducible matrix containing +Inf to verify that non-finite values are clearly reported instead of silently producing NA estimates. Done means the user-facing behavior identifies the invalid log_lik values and the existing example no longer completes without that information.