missing error messages after failed initialization

Summary:

Error messages get lost after failed initialization.

Reproducible Steps:

In RStan 2.18.1 on Mac OS X, this command

stan(model_code = 'parameters { real x; } model { reject("foo"); }', chains=1, iter=10)

fails initialization.

Current Output:

15 verbose warning messages, then

SAMPLING FOR MODEL '61b0f66ecff6751e01f0b5e389249dfd' NOW (CHAIN 1).
Chain 1: Initialization between (-2, 2) failed after 100 attempts. 
[1] "Error in sampler$call_sampler(args_list[[i]]) : Initialization failed."
error occurred during calling the sampler; sampling not done

Expected Output:

Something that includes the error message, which here should be "foo".

RStan Version:

> sessionInfo()
R version 3.5.0 (2018-04-23)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.6

Matrix products: default
BLAS: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rstan_2.18.1       StanHeaders_2.18.0 ggplot2_2.2.1     

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.18       magrittr_1.5       munsell_0.4.3      colorspace_1.3-2  
 [5] R6_2.2.2           rlang_0.2.1        plyr_1.8.4         tools_3.5.0       
 [9] parallel_3.5.0     pkgbuild_1.0.2     grid_3.5.0         gtable_0.2.0      
[13] loo_2.0.0          cli_1.0.0          matrixStats_0.54.0 lazyeval_0.2.1    
[17] assertthat_0.2.0   tibble_1.4.2       crayon_1.3.4       processx_3.2.0    
[21] gridExtra_2.3      callr_3.0.0        codetools_0.2-15   base64enc_0.1-3   
[25] ps_1.2.0           inline_0.3.15      compiler_3.5.0     pillar_1.2.3      
[29] scales_0.5.0       prettyunits_1.0.2  stats4_3.5.0      

Operating System:

OS X 10.13.6 (High Sierra)