* lisp/emacs-lisp/smie.el (smie-bnf-precedence-table): Improve error message.

* lisp/emacs-lisp/smie.el (smie-bnf-precedence-table): Improve error message.
(smie-debug--prec2-cycle, smie-debug--describe-cycle): New functions.
(smie-prec2-levels): Use them to better diagnose precedence cycles.
(smie-blink-matching-check): Don't signal a mismatch if car is t.
(smie-blink-matching-open): Rewrite to remove assumptions, so that
something like "." can also be a closer.
(smie--associative-p, smie-indent--hanging-p, smie-indent--bolp)
(smie-indent--offset, smie-indent--offset-rule, smie-indent--column):
Rename internal functions to use "--".  Update callers.