Don't error out on invalid literal key substitutions

Don't error out on invalid literal key substitutions

It would be backwards-incompatible to error out on invalid literal key
substitutions.  Consider this docstring fragment, where "\\`" should
have been escaped but wasn't:

    "Example value: \"\\(?:\\`[#.]\\)\\|\\(?:[#~]\\'\\)\"."

If we error out, we can't display this docstring at all.  However, it
is clearly better to display something in such cases, even if
suboptimal, than refusing to display anything at all.

* lisp/help.el (substitute-command-keys): Don't error out on invalid
literal key substitutions: just ignore them instead.
* test/lisp/help-tests.el
(help-tests-substitute-command-keys/literal-key-sequence-errors):
Delete test.
(help-tests-substitute-command-keys/literal-key-sequence-ignore-invalid):
New test.