[Haml] Make html2haml fully respect --trace.

doc-src/HAML_CHANGELOG.md

@@ -3,6 +3,11 @@
 * Table of contents
 {:toc}
 
+## 2.2.6 (Unreleased)
+
+* Made the error message when unable to load a dependency for html2haml
+  respect the `--trace` option.
+
 ## [2.2.5](http://github.com/nex3/haml/commit/2.2.5)
 
 * Got rid of trailing whitespace produced when opening a conditional comment

lib/haml/exec.rb

@@ -364,7 +364,8 @@ END
           require 'haml/html'
         rescue LoadError => err
           dep = err.message.scan(/^no such file to load -- (.*)/)[0]
-          puts "Required dependency #{dep} not found!"
+          raise err if @options[:trace] || dep.nil? || dep.empty?
+          $stderr.puts "Required dependency #{dep} not found!\n  Use --trace for backtrace."
           exit 1
         end
       end