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))
-      local out
-      do
-        local _with_0 = p:read("*a")
-        p:close()
-        out = _with_0
-      end
-      return pandoc.RawBlock("html", out)
+      cmd = cmd .. " -x"
+      lexer = "moonscript.py"
     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
-        return block
-      else
-        return pandoc.RawBlock("html", out)
+      lexer = label or "text"
+    end
+    local f = io.popen(cmd:format(lexer, fname, errfname))
+    local out
+    do
+      local _with_0 = f:read("*a")
+      f:close()
+      out = _with_0
+    end
+    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
 }