checkend – Extend "improperly closed environment" messages

When an en­vi­ron­ment is left open, LaTeX gives an er­ror at the end of the doc­u­ment. How­ever it only in­forms about the first of them, while the rest are shown with mean­ing­less er­rors:
    (\end oc­curred in­side a group at level N) This pack­age re­places these er­rors with more use­ful mes­sages which show which en­vi­ron­ments (in re­verse or­der) were not closed. There are no user macros: just use the pack­age.
The pack­age is part of the be­zos bun­dle.