Build Summoner with -Werror on CI

So we can catch all warnings earlier. Due to some challenges with specifying `-Werror` for `cabal` only on CI (see #173 for more details), I propose to add `-Werror` as a CLI argument to Stack.

Ideally, we also want to build all our generated test projects with `-Werror` to be sure that we don't generate projects with warnings, but that's a separate issue I guess.

Read issue #173 for the challenges around passing -Werror through cabal, then inspect Stack's CLI argument handling and the CI build configuration. Done means CI builds Summoner with -Werror enabled and fails on compiler warnings.

Written by the indexing model from the issue text.