Provide some backtrace info when a thor file fails to load.

By default the first line will be sent to stderr. If the --debug flag is
passed then the whole backtrace will be printed.

lib/thor/runner.rb

@@ -39,7 +39,7 @@ def method_missing(meth, *args)
   def install(name)
     initialize_thorfiles
 
-    # If a directory name is provided as the argument, look for a 'main.thor' 
+    # If a directory name is provided as the argument, look for a 'main.thor'
     # task in said directory.
     begin
       if File.directory?(File.expand_path(name))
@@ -139,7 +139,7 @@ def installed
   end
 
   desc "list [SEARCH]", "List the available thor tasks (--substring means .*SEARCH)"
-  method_options :substring => :boolean, :group => :string, :all => :boolean
+  method_options :substring => :boolean, :group => :string, :all => :boolean, :debug => :boolean
   def list(search="")
     initialize_thorfiles
 
@@ -199,7 +199,7 @@ def self.exit_on_failure?
     #
     def initialize_thorfiles(relevant_to=nil, skip_lookup=false)
       thorfiles(relevant_to, skip_lookup).each do |f|
-        Thor::Util.load_thorfile(f) unless Thor::Base.subclass_files.keys.include?(File.expand_path(f))
+        Thor::Util.load_thorfile(f, nil, options[:debug]) unless Thor::Base.subclass_files.keys.include?(File.expand_path(f))
       end
     end
 

lib/thor/util.rb

@@ -147,13 +147,18 @@ def self.find_class_and_task_by_namespace(namespace, fallback = true)
     # Receives a path and load the thor file in the path. The file is evaluated
     # inside the sandbox to avoid namespacing conflicts.
     #
-    def self.load_thorfile(path, content=nil)
+    def self.load_thorfile(path, content=nil, debug=false)
       content ||= File.binread(path)
 
       begin
         Thor::Sandbox.class_eval(content, path)
       rescue Exception => e
         $stderr.puts "WARNING: unable to load thorfile #{path.inspect}: #{e.message}"
+        if debug
+          $stderr.puts *e.backtrace
+        else
+          $stderr.puts e.backtrace.first
+        end
       end
     end