block_trust_region MEX: improve treatment of non-square blocks in Dulmage-Mendelsohn decomposition

– before erroring out, check whether the residuals for the block are already
  zero (in which case, move to next block)
– improve error message that is printed otherwise

Note that trying to solve under-determined blocks (as in dynare_solve.m) would
require too many changes in the existing code, so let’s leave it out.

Closes: #1851