Improve swap-deps error handling for deleted directories (#47)

When swap-deps encounters a missing directory path (e.g., a deleted
Conductor workspace), it now provides a helpful error message that:

- Clearly states which path doesn't exist
- Explains common causes (outdated .swap-deps.yml, deleted repo)
- Suggests fixes (update config or use --restore)

This makes the tool more user-friendly when dealing with stale
configuration files that reference non-existent paths.

Author: justin808

lib/demo_scripts/gem_swapper.rb

@@ -519,7 +519,15 @@ def validate_local_paths!
       gem_paths.each do |gem_name, path|
         next if File.directory?(path)
 
-        raise Error, "Local path for #{gem_name} does not exist: #{path}"
+        error_msg = "Local path for #{gem_name} does not exist: #{path}\n\n"
+        error_msg += "This usually means:\n"
+        error_msg += "  1. The path in .swap-deps.yml is outdated\n"
+        error_msg += "  2. You moved or deleted the local repository\n\n"
+        error_msg += "To fix:\n"
+        error_msg += "  - Update .swap-deps.yml with the correct path\n"
+        error_msg += '  - Or use --restore to restore original dependencies'
+
+        raise Error, error_msg
       end
     end