handle bad checksums without erring out

[anish]

 -> Found google-chrome-unstable_current_amd64.deb
==> Validating source files with md5sums...
  google-chrome-unstable_current_amd64.deb ... FAILED
==> ERROR: One or more files did not pass the validity check!
 error: /usr/share/lua/5.1/clydelib/aur.lua:410: Build failed. makepkg returned 256

I shouldn't see the lua message, nor do I need to see makepkg return values unless I'm in some sort of verbose mode. Clyde should be able to quit or continue gracefully in this situation