Crash on :aot [does.not.exist]; also regression test; Fixes #1205

We want missing namespaces in :aot to not be silently ignored.
Implemented by removing the file-existence check from the function
that determines which namespaces need to be compiled.

This means we won't get the error if you compile an existing namespace
and subsequently delete the *.clj file. Oh well.
Merge pull request #1206 from fredericksgary/issue-1205

Crash on :aot [does.not.exist]; also regression test; Fixes #1205