Tweak runtests to be more helpful

If nose isn't installed, then runtests.sh says it can't find nosetests
and exits, but doesn't tell you what you need to do to fix the situation.
This fixes that.

runtests.sh

@@ -23,7 +23,8 @@ elif which nosetests > /dev/null; then
     echo "Using nosetests from \$PATH";
     export NOSETESTS="nosetests";
 else
-    echo "No nosetests found, exiting! X_X";
+    echo "nosetests not found.  X_X";
+    echo "Please install 'nose'.  Exiting.";
     exit 1
 fi