Better report compilation errors during build

Cakefile

@@ -57,20 +57,27 @@ write_chosen_javascript = (filename, body) ->
 // This file is generated by `cake build`, do not edit it by hand.
 #{body}
 """
+  console.log "Wrote #{filename}"
 
 # Build Chosen.
 #
 task 'build', 'build Chosen from source', build = (cb) ->
+  file_name = null; file_contents = null
+  try
     for javascript, sources of javascripts
       code = ''
       for source in sources
-      code += CoffeeScript.compile "#{fs.readFileSync source}"
+        file_name = source
+        file_contents = "#{fs.readFileSync source}"
+        code += CoffeeScript.compile file_contents
       write_chosen_javascript javascript, code
       unless process.env.MINIFY is 'false'
         write_chosen_javascript javascript.replace(/\.js$/,'.min.js'), (
           uglify.gen_code uglify.ast_squeeze uglify.ast_mangle parser.parse code
         )
     cb() if typeof cb is 'function'
+  catch e
+    print_error e, file_name, file_contents
  
 task 'watch', 'watch coffee/ for changes and build Chosen', ->
   console.log "Watching for changes in coffee/"
@@ -118,6 +125,28 @@ untag_release = (e) ->
 push_repo = (args=[], cb, cb_err) ->
   run 'git', ['push'].concat(args), cb, cb_err
 
+print_error = (error, file_name, file_contents) ->
+  line = error.message.match /line ([0-9]+):/
+  if line && line[1] && line = parseInt(line[1])
+    contents_lines = file_contents.split "\n"
+    first = if line-4 < 0 then 0 else line-4
+    last  = if line+3 > contents_lines.size then contents_lines.size else line+3
+    console.log "Error compiling #{file_name}. \"#{error.message}\"\n"
+    index = 0
+    for line in contents_lines[first...last]
+      index++
+      line_number = first + 1 + index
+      console.log "#{(' ' for [0..(3-(line_number.toString().length))]).join('')} #{line}"
+  else
+    console.log """
+Error compiling #{file_name}:
+  
+  #{error.message}
+
+"""
+
+
+
 task 'release', 'build, tag the current release, and push', ->
   console.log "Trying to tag #{version_tag()}..."
   with_clean_repo ->