Print compile error stacktraces as they occur

While attempting to compile a main .clj file to a class, to create an uberjar with "lein uberjar", I ran into a mysterious compilation error that only printed "Compilation failed: Subprocess failed" with no further information. (The program works fine from the REPL.) 

I modified Leiningen to print a stack trace when compile errors occur, which gave me the information I needed to debug the problem. I submit the attached modification in case anyone else finds this useful.