Make sure nvm ls foo returns a nonzero exit code when a version is not found.

Fixes #440.

test/fast/Running "nvm ls foo" should return a nonzero exit code when not found

@@ -0,0 +1,7 @@
+ #!/bin/sh
+
+. ../../nvm.sh
+
+nvm ls nonexistent_version
+[ "$?" = "3" ]
+