report where cached compiled model was obtained from if `verbose=TRUE`

Summary:

Currently, even if verbose=TRUE, the function stan_model does not report whether a precompiled model was found, or from where. This makes debugging segfaults induced by loading a previously built but incompatible model difficult.

Description:

I encountered this after I had run some code with auto_write=TRUE on our cluster with R 3.6.3, then later ran the same code with R 4.0.2; the result was a segfault, and - foolishly - it took me quite a while to figure out that the problem was a .rds file. In the course of tracking this down, I had set auto_write=FALSE and then run it with stan_model(..., verbose=TRUE), expecting it would tell me if it was obtaining a cached model, but it did not. Additional verbosity wouldn't have avoided the problem entirely (I needed to rtm), but it would have helped.

Reproducible Steps:

Run

library(rstan)
rstan_options(auto_write = TRUE)

schools_data <- list(
  J = 8,
  y = c(28,  8, -3,  7, -1,  1, 18, 12),
  sigma = c(15, 10, 16, 11,  9, 11, 10, 18)
)
model <- stan_model(file="schools.stan", verbose=TRUE)
fit <- stan( file = "schools.stan", data = schools_data,
             chains = 1, warmup = 100, iter = 200, cores = 1, refresh = 0)

twice; once under R 3.6.3 and then again under R 4.0.2. The result is a segfault (of course), but more importantly, stan_model only outputs:

TRANSLATING MODEL 'schools' FROM Stan CODE TO C++ CODE NOW.
successful in parsing the Stan model 'schools'.

 *** caught segfault ***
address 0x2aaabf663008, cause 'invalid permissions'

Expected Output:

Perhaps this expression could be extended to include something like

if (verbose) message(sprintf("reloading compiled Stan model from %s", file.rds))

Also, this line might include

if (verbose) messge(sprintf("reloading compiled Stan model from stanfit object %s", i))

and this line might include

if (verbose) messge(sprintf("reloading compiled Stan model from stanmodel object %s", i))

Start in rstan/rstan/R/rstan.R at the stan_model cache-loading paths referenced in the issue, including the branches for cached files and existing stanfit or stanmodel objects. Run the provided schools.stan example with verbose=TRUE and confirm that each cached-model source is reported before loading.