Handle errors in load_plugin(s) more graceful

This patch tries to address https://rt.cpan.org/Ticket/Display.html?id=88562
When a plugin fails to load, load_plugin returns false, and prints a one liner warning. load_plugins returns true if any of the plugins where loaded successfully.
Tested with Devel::REPL when used with plugin LexEnv when its prereq Lexical::Persistence is not available. Unittests added.
Please review!