This commit addresses #8051 by fixing

 - Incorrect column indices reported in error messages for
   single-line and multi-line input,

 - incorrect line numbers reported in error messages for
   expressions entered in multi-line input, and

 - inhibiting the confusing interaction between `:{` and `:set +m`
   causing the triggering of implicit multi-line continuation
   mode right after `:}` terminates the multi-line entry block.