Logging capability for debugging purposes

[rpkyle]

In Dash for R and Dash for Python, we currently provide logging output for requests and responses. In R, it looks like this (the colours are missing, but successful requests are green, failed requests are red, and warnings are yellow):

> app$run_server()
Fire started at 127.0.0.1:8050
start: 127.0.0.1:8050
request: 127.0.0.1 - ID_127.0.0.1 [25/Apr/2020:14:47:38 -0400] "GET / HTTP/1.1" 200 591 "" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.122 Safari/537.36"
error: Script or CSS dependencies with NULL href fields must include a file path, dependency name, and R package name.
request: 127.0.0.1 - ID_127.0.0.1 [25/Apr/2020:14:47:40 -0400] "GET /_dash-component-suites/dash_core_components/dash_core_components.v1_9_0m1586465028.min.js?v=1.9.0&m=1586465028 HTTP/1.1" 500 0 "http://127.0.0.1:8050/" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.122 Safari/537.36"

In addition to a logging facility, it would be good to trap errors as warnings, so that the backend doesn't halt when a (recoverable) problem is encountered. Particularly in multi-user contexts, this prevents a simple/transient problem from taking down the application completely.