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