more descriptive LoadError for spork/ext/ruby-debug

if ruby-debug not installed and the project loads spork/ext/ruby-debug, then it shows an error saying 'ruby-debug' not installed.

This can confuse the user, as they required 'spork/ext/ruby-debug' in the project, not ruby-debug.  Why didn't spork install ruby-debug for me?

Well, the answer is because spork/ext/ruby-debug is optional.  This new error should make that clear.

lib/spork/ext/ruby-debug.rb

@@ -1,7 +1,9 @@
-require 'ruby-debug'
 require 'socket'
 require 'forwardable'
 
+begin
+require 'ruby-debug'
+
 # Experimental!
 
 class SporkDebugger
@@ -143,3 +145,6 @@ def wait_for_connection
 
 Spork.prefork { SporkDebugger.run } if Spork.using_spork?
 
+rescue LoadError
+  raise Loaderorr, "Your project has loaded spork/ext/ruby-debug, which relies on the ruby-debug gem. It appears that ruby-debug is not installed. Please install it."
+end