Provide  actionable error message in RegexpTagger
Provide  actionable error message in RegexpTagger

The RegexpTagger error messages are obscure when a regexp has an error.
This is because re messages may be obscure too.

To help with tracing the cause or regex errors, we now catch regex
compilation errors when a RegexpTagger is contrsuctor and raise a new
exception that contains the required diagnostic information with the
error message, the faulty regex and its corresponding tag.

The RegexpTagger.decode_json_obj has been streamlined also to avoid code
duplication and use the default constructor instead.

Signed-off-by: Philippe Ombredanne <ca95c4a6a4931f366cbdaf5878c5016609417d37@nexb.com>
nltk/nltk,nltk/nltk,nltk/nltk