Robustify the plugin require

I was getting trouble with both pry-editline and pry-syntax-hacks, while
running with a newly-compiled-Ruby with GNU Readline on a Mac (the
default Readline was working OK with these. There could be more
investigation about the error, but for now, at least pry boots with a
handy error instead of a total failure).

lib/pry/plugins.rb

@@ -50,6 +50,8 @@ class Pry
         rescue LoadError => e
           warn "Found plugin #{gem_name}, but could not require '#{gem_name}'"
           warn e
+        rescue => e
+          warn "require '#{gem_name}' # Failed, saying: #{e}"
         end
 
         self.active = true