fix and cleanup wildcard error msgs, refactor/cleanup generate_dasquery()

**fix and cleanup wildcard error msgs**
* clean logic to handle wildcard exceptions
  - earlier "The pattern you specified did not match any datasets in DAS cache." was caught as a generic exception and thus initiating KWS.
* rename helper() into error_msg()
* separate templates for html and text (CLI) modes also for das_ambiguous
* in das_wildcard_err, use same name 'msg' as in das_ambiguous for the error message

**visual cleanup of generate_dasquery()**
* PEP8
* extract check_daskey(dasquery) which checks if given dasquery contain a valid daskey