Let warnings cause the build to fail but not to terminate #3627

Subject: sphinx-build should be able to treat warnings as errors without immediately terminating the build

Problem

sphinx-build should gain a switch that allows warnings to cause a return value !=0 (i.e., failed build) without immediately terminating the build, so that as all warnings and the first "true" error all get reported.
This is useful for CI on docs which are relatively long to build (e.g. matplotlib), which right now need to be re-run after each warning is fixed to get the next warning (of course, one could locally just not use the -W switch, but it may be tricky for contributors to build docs locally as we test a variety of versions of python and numpy and have non-python dependencies).

Procedure to reproduce the problem

sphinx-build -W

on a doc that generates warnings

Environment info

any