Investigate unmatched returns found by Dialyzer

[jchristgit]

As we all know, Dialyzer is never wrong. #508 attempts to introduce the new
-Wunmatched_return flag to check for return values we are properly dealing
with, and it finds a few of it in this job
log.
We should figure out what Dialyzer has found, fix it and merge the changes to
enable the warning accordingly.