qibuild/swig.cmake: fail eary if swig executable is not found
qibuild/swig.cmake: fail eary if swig executable is not found

Else, when swig is not found, you get something like:
  Swig source ...
  'SWIG_EXECUTABLE-NOTFOUND' : no such file or directory
at build time rather than having an error at configure time.

aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild