Add verbosity to download output and errors

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR updates the file downloading functions to respect the quiet argument from an install_cmdstan() call, and also to print the actual error message if the download fails.

This will hopefully help with debugging issues like #731

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #745 (baadb05) into master (b96a62a) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head baadb05 differs from pull request most recent head 2a6d2c9. Consider uploading reports for the commit 2a6d2c9 to get more accurate results

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
+ Coverage   88.30%   88.36%   +0.06%     
==========================================
  Files          12       12              
  Lines        4077     4082       +5     
==========================================
+ Hits         3600     3607       +7     
+ Misses        477      475       -2     

Impacted Files	Coverage Δ
R/install.R	62.89% <100.00%> (+0.88%)	⬆️

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[jgabry]

Thanks, good idea!