Make incompatible feature errs more user-friendly

- Use JERR_NOTIMPL ("Not implemented yet") rather than JERR_NOT_COMPILED
  ("Requested feature was omitted at compile time") to indicate that
  arithmetic coding is incompatible with Huffman table optimization.
  This is more consistent with other parts of the libjpeg API code.
  JERR_NOT_COMPILED is typically used to indicate that a major feature
  was not compiled in, whereas JERR_NOTIMPL is typically used to
  indicate that two features were compiled in but are incompatible with
  each other (such as, for instance, two-pass color quantization and
  partial image decompression.)

- Change the text of JERR_NOTIMPL to "Requested features are
  incompatible".  This is a more accurate description of the situation.
  "Not implemented yet" implies that it may be possible to support the
  requested combination of features in the future, but that is not true
  in most of the cases where JERR_NOTIMPL is used.

Fixes #567

jcarith.c

@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Developed 1997-2009 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2015, 2018, D. R. Commander.
+ * Copyright (C) 2015, 2018, 2021, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -836,7 +836,7 @@ start_pass(j_compress_ptr cinfo, boolean gather_statistics)
      * We are fully adaptive here and need no extra
      * statistics gathering pass!
      */
-    ERREXIT(cinfo, JERR_NOT_COMPILED);
+    ERREXIT(cinfo, JERR_NOTIMPL);
 
   /* We assume jcmaster.c already validated the progressive scan parameters. */