uninstall -f ignores nonexisting packages.

lib/spack/spack/cmd/uninstall.py

@@ -66,6 +66,7 @@ def uninstall(parser, args):
             tty.die(*args)
 
         if len(matching_specs) == 0:
+            if args.force: continue
             tty.die("%s does not match any installed packages." % spec)
 
         for s in matching_specs: