Added detection of missing classes directory and missing method bodies

If a user has an IDE configured to create the .class file at other locations than $PWD/bin or $PWD/build/classes, the tests will fail, but no information is given to the user as to why.

Patch 4c767f requires one of those directories to be present, and aborts the testing if they are missing.
Patch cc9dc20 checks if a target has no body and returns an Identity function if so.