Add failure error message to pygments lua-filter

pygments.lua

@@ -2,33 +2,40 @@ return {
   CodeBlock = function(block)
     local label
     label = block.classes[1]
-    local cmd = "pygmentize -f html -O style=dracula,wrapcode,classprefix=py- -l %s %s"
-    local fname = os.tmpname()
+    local fname, errfname = os.tmpname(), os.tmpname()
+    local cmd = "pygmentize -f html -O style=dracula,wrapcode,classprefix=py- -l %s %s 2>%s"
     do
       local _with_0 = io.open(fname, "w")
       _with_0:write(block.text)
       _with_0:close()
     end
+    local lexer
     local _exp_0 = label
     if "moon" == _exp_0 or "moonscript" == _exp_0 then
-      local p = io.popen((cmd .. " -x"):format("moonscript.py", fname))
+      cmd = cmd .. " -x"
+      lexer = "moonscript.py"
+    else
+      lexer = label or "text"
+    end
+    local f = io.popen(cmd:format(lexer, fname, errfname))
     local out
     do
-        local _with_0 = p:read("*a")
-        p:close()
+      local _with_0 = f:read("*a")
+      f:close()
       out = _with_0
     end
-      return pandoc.RawBlock("html", out)
-    else
-      local status, handle = pcall(function()
-        return io.popen(cmd:format(label or "text", fname))
-      end)
-      local out = handle:read("*a")
-      if out:len() == 0 then
+    if out == "" then
+      local e = io.open(errfname, "r")
+      local err
+      do
+        local _with_0 = e:read("*a")
+        e:close()
+        err = _with_0
+      end
+      assert(err:match("Error: no lexer for alias '" .. tostring(label) .. "' found"), "Failed to parse pygmentize result, is pygments installed?")
       return block
     else
       return pandoc.RawBlock("html", out)
     end
   end
-  end
 }

pygments.moon

@@ -1,26 +1,33 @@
 CodeBlock: (block) ->
   { label } = block.classes
-  cmd = "pygmentize -f html -O style=dracula,wrapcode,classprefix=py- -l %s %s"
-  fname = os.tmpname!
+  fname, errfname = os.tmpname!, os.tmpname!
+  cmd = "pygmentize -f html -O style=dracula,wrapcode,classprefix=py- -l %s %s 2>%s"
 
   with io.open fname, "w"
     \write block.text
     \close!
 
-  switch label
+  lexer = switch label
     when "moon", "moonscript"
-      p = io.popen (cmd .. " -x")\format "moonscript.py", fname
-      out = with p\read"*a"
-        p\close!
-      -- syntax highlight with pygments
-      pandoc.RawBlock "html", out
+      cmd = cmd .. " -x"
+      "moonscript.py"
     else
-      status, handle = pcall -> io.popen cmd\format label or "text", fname
-      out = handle\read"*a"
+      label or "text"
+
+  f = io.popen cmd\format lexer, fname, errfname
+
+  out = with f\read"*a"
+    f\close!
+
+  if out == ""
+    e = io.open errfname, "r"
+
+    err = with e\read"*a"
+      e\close!
+
+    assert err\match("Error: no lexer for alias '#{label}' found"),
+      "Failed to parse pygmentize result, is pygments installed?"
 
-      if out\len! == 0
-        -- syntax highlight with pandoc
     block
   else
-        -- syntax highlight with pygments
     pandoc.RawBlock "html", out