Fix #4856: Don't rescue LoadError or bundler load errors for Bundler.

When it comes to bundler it's smart enough to know what to require, and in casees it's not, it's smart enough to accept :require.  In most cases when bundler has a LoadError (or otherwise) it's because there is a problem inside of the Gem itself and when this happens, Jekyll will happily let that error slip when it shouldn't, resulting in a badly placed error that is actually wrong. This corrects that so errors can surface properly.

lib/jekyll/plugin_manager.rb

@@ -30,16 +30,16 @@ module Jekyll
     def self.require_from_bundler
       if !ENV["JEKYLL_NO_BUNDLER_REQUIRE"] && File.file?("Gemfile")
         require "bundler"
-        Bundler.setup # puts all groups on the load path
-        required_gems = Bundler.require(:jekyll_plugins) # requires the gems in this group only
+
+        Bundler.setup
+        required_gems = Bundler.require(:jekyll_plugins)
         Jekyll.logger.debug("PluginManager:", "Required #{required_gems.map(&:name).join(', ')}")
         ENV["JEKYLL_NO_BUNDLER_REQUIRE"] = "true"
+
         true
       else
         false
       end
-    rescue LoadError, Bundler::GemfileNotFound
-      false
     end
 
     # Check whether a gem plugin is allowed to be used during this build.