[API Change] Add new error constructor PandocNoScriptingEngine.

The exit code for this error is 89.

MANUAL.txt

@@ -1539,6 +1539,7 @@ Nonzero exit codes have the following meanings:
    67 PandocSyntaxMapError
    83 PandocFilterError
    84 PandocLuaError
+   89 PandocNoScriptingEngine
    91 PandocMacroLoop
    92 PandocUTF8DecodingError
    93 PandocIpynbDecodingError

src/Text/Pandoc/Error.hs

@@ -53,6 +53,7 @@ data PandocError = PandocIOError Text IOError
                  | PandocXMLError Text Text
                  | PandocFilterError Text Text
                  | PandocLuaError Text
+                 | PandocNoScriptingEngine
                  | PandocCouldNotFindDataFileError Text
                  | PandocCouldNotFindMetadataFileError Text
                  | PandocResourceNotFound Text
@@ -117,6 +118,8 @@ renderError e =
     PandocFilterError filtername msg -> "Error running filter " <>
         filtername <> ":\n" <> msg
     PandocLuaError msg -> "Error running Lua:\n" <> msg
+    PandocNoScriptingEngine -> "This version of pandoc has been compiled " <>
+                               "without Lua support."
     PandocCouldNotFindDataFileError fn ->
         "Could not find data file " <> fn
     PandocCouldNotFindMetadataFileError fn ->
@@ -196,6 +199,7 @@ handleError (Left e) =
       PandocSyntaxMapError{} -> 67
       PandocFilterError{} -> 83
       PandocLuaError{} -> 84
+      PandocNoScriptingEngine -> 89
       PandocMacroLoop{} -> 91
       PandocUTF8DecodingError{} -> 92
       PandocIpynbDecodingError{} -> 93

src/Text/Pandoc/Scripting.hs

@@ -15,10 +15,12 @@ module Text.Pandoc.Scripting
   )
 where
 
+import Control.Monad.Except (throwError)
 import Control.Monad.IO.Class (MonadIO)
 import Data.Text (Text)
 import Text.Pandoc.Definition (Pandoc)
 import Text.Pandoc.Class.PandocMonad (PandocMonad)
+import Text.Pandoc.Error (PandocError (PandocNoScriptingEngine))
 import Text.Pandoc.Filter.Environment (Environment)
 import Text.Pandoc.Options (ReaderOptions, WriterOptions)
 import Text.Pandoc.Sources (Sources)
@@ -45,9 +47,9 @@ noEngine :: ScriptingEngine
 noEngine = ScriptingEngine
   { engineName = "none"
   , engineApplyFilter = \_env _args _fp _doc ->
-      error "Custom filters are not supported."
+      throwError PandocNoScriptingEngine
   , engineReadCustom = \_fp _ropts _sources ->
-      error "Custom readers are not supported."
+      throwError PandocNoScriptingEngine
   , engineWriteCustom = \_fp _wopts _doc ->
-      error "Custom writers are not supported."
+      throwError PandocNoScriptingEngine
   }