Add error handling to bundler downgrade

`set -e` stops the script when a *simple command* fails.
If the command is part of an `||`-expression, the script continues.

It seems unessential that we fail to uninstall `bundler` from the global
gemset, the build is green after all.

Author: pirj

script/downgrade_bundler_on_old_rails

@@ -11,9 +11,11 @@ if ruby -e "exit(ENV['RAILS_VERSION'].scan(/\d+\.\d+.\d+/)[0].to_f < 5)"; then
 
   gem uninstall -aIx bundler || echo "Warning error occured removing bundler via gem"
 
+  echo "Does it fail to execute the script further here?"
+
   # this only works on Ruby 2.3 which is luckily the version we need to fix
   if ruby -e "exit(RUBY_VERSION.to_f == 2.3)"; then
-    rvm @global do gem uninstall -aIx bundler
+    rvm @global do gem uninstall -aIx bundler || echo "Warning: error occured removing bundler from the global RVM gemset"
   fi
 
   gem install bundler -v '1.17.3'