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

Hoping somebody who's at least seen the `compile` code before can
glance at this to make sure it doesn't obviously break anything that
isn't tested.

Original commit msg:

> 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.