rpmbuild: less verbose RuntimeError(s)
    
    It only prolonged the error output unnecessarily:
    Traceback (most recent call last):
      File "/usr/bin/copr-rpmbuild", line 129, in main
        action(args, config)
      File "/usr/bin/copr-rpmbuild", line 262, in build_rpm
        builder.run()
      File "/usr/lib/python3.7/site-packages/copr_rpmbuild/builders/mock.py", line 40, in run
        self.produce_srpm(spec, self.sourcedir, self.resultdir)
      File "/usr/lib/python3.7/site-packages/copr_rpmbuild/builders/mock.py", line 97, in produce_srpm
        raise RuntimeError("Build failed")
    RuntimeError: Build failed
    
    Now it is just:
    ERROR: Copr build error: Mock build failed
    
    Relates: #1285