extra error checking for Find MPI Targets

`BLT_USE_FIND_MPI_TARGETS` uses `MPI::MPI_CXX` but doesn't explicitly check it exists.

If it's not there a confusing error message is a likely outcome.

Locate the CMake logic for BLT_USE_FIND_MPI_TARGETS and inspect how it uses MPI::MPI_CXX. Add an explicit check that the target exists so configuration reports a clear error, then verify the affected CMake configuration path succeeds when the target is available and fails clearly when it is not.

Written by the indexing model from the issue text.