Add fail_if_missing option

[Polexy]

In case of some deps missing - relx successful build release

rebar3 release as prod
===> Verifying dependencies...
===> Analyzing applications...
...
===> There are missing function calls in the release.
===> Make sure all applications needed at runtime are included in the release.
...
===> Release successfully assembled: _build/default/rel/myapp


echo $?
0

so in case of build in ci/cd it may be useful to fail command if missing deps in relx