Add failure error message to pygments lua-filter

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"
 
-      if out\len! == 0
-        -- syntax highlight with pandoc
-        block
-      else
-        -- syntax highlight with pygments
-        pandoc.RawBlock "html", out
+  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?"
+
+    block
+  else
+    pandoc.RawBlock "html", out