Work on recognizing, throwing, and displaying errors that come from the shell.

* [ERROR_EXT,NEW] Added exit_code_reason/2 and process_error/2.
* [HTML_AP_TERM,NEW] New module for generating HTML descriptions of AP terms.
* [HTML_PL_TERM,EXT] Greatly extended this module, added public predicates that interface with HTML_AP_TERM. Rewrote some error-specific DCG rules to be applicable more widely.
* [RUN_EXT,NEW] Added exit_code_handler/2 for handlign shell exit codes. Simplified error throwing upon shell errors overall.
* [SHELL_EXT,SC] Removed old shell status code, superseded by exit_code_handler/2.

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