Revert 'Consistent use of "failure" instead of "failure occurs".'

This reverts commit 15eb27acf8aacdea8d4e9a4c92c49d93e372f6bf.

"Failure occurs if ..." is more grammatical than "Failure if ...".
There are some existing occurrences of the latter that still need to
be fixed.
Fix some unnecessarily terse "Failure if <some condition>" clauses

Better is "Failure occurs if <some condition>". Such sentences have
verbs.
Clean up some doubly-nested LaTeX environments for the sake of Emacs.

The problem is not the double-nesting per se, but the use of

    \begin{foo}\begin{bar}
    ...
    \end{bar}\end{foo}

which, when bar is "alltt" or "verbatim" seems to cause AucTeX to get
into a terrible font-locking confusion.

Also remove some unnecessary local definitions of \bk to be a teletype
backslash when our commands.tex include-file has \bs for this
character already.