if not all tests passed, indicate to the calling program that there was an error

Loosely based on commit 089e560a2668008094554103d10cc02a2d60204e in master branch
"make clean" should use "rm -f" so that there are not errors when
the files being deleted do not exist.

See commit 08962f5374aa9f8ac214ff09a563e7eb59ad9aa0 on master branch
Build libzerocoin.a