[Sass] Print the Sass backtrace on error for the sass executable.

doc-src/SASS_CHANGELOG.md

@@ -12,6 +12,10 @@ especially when importing files.
 All imports will now show up in the Ruby backtrace,
 with the proper filename and line number.
 
+In addition, when the `sass` executable encounters an error,
+it now prints the filename where the error occurs,
+as well as a backtrace of Sass imports.
+
 ## [2.2.3](http://github.com/nex3/haml/commit/2.2.3)
 
 Sass 2.2.3 prints line numbers for warnings about selectors

lib/haml/exec.rb

@@ -264,7 +264,7 @@ END
         output.close() if output.is_a? File
       rescue ::Sass::SyntaxError => e
         raise e if @options[:trace]
-        raise "Syntax error on line #{get_line e}: #{e.message}"
+        raise e.sass_backtrace_str("standard input")
       end
     end
 

lib/sass/error.rb

@@ -96,6 +96,22 @@ module Sass
       return nil if super.nil?
       sass_backtrace.map {|h| "#{h[:filename] || "(sass)"}:#{h[:line]}"} + super
     end
+
+    # Returns a string representation of the Sass backtrace.
+    #
+    # @param default_filename [String] The filename to use for unknown files
+    # @see #sass_backtrace
+    # @return [String]
+    def sass_backtrace_str(default_filename = "an unknown file")
+      msg = "Syntax error on line #{sass_line}" +
+        " of #{sass_filename || default_filename}" +
+        ": #{message}"
+      sass_backtrace[1..-1].each do |entry|
+        msg << "\n        from line #{entry[:line]}" +
+          " of #{entry[:filename] || default_filename}"
+      end
+      msg
+    end
   end
 
   # The class for Sass errors that are raised due to invalid unit conversions