message: Providing more informative error message regarding Gmsh attributes
diff: @@ -1540,7 +1540,11 @@ void Mesh::ReadGmshMesh(std::istream &input, int &curved, int &read_gf)
  // non-positive attributes are not allowed in MFEM
  if (phys_domain <= 0)
  {
  - MFEM_ABORT("Non-positive element attribute in Gmsh mesh!");
  + MFEM_ABORT("Non-positive element attribute in Gmsh mesh!\n"
  + "Use \"Physical Curve\", \"Physical Surface\","
  + " or \"Physical Volume\" to set attributes "
  + "for all curves, surfaces, or volumes in your "
  + "Gmsh geometry.");
  }
  // initialize the mesh element
  @@ -1760,7 +1764,11 @@ void Mesh::ReadGmshMesh(std::istream &input, int &curved, int &read_gf)
  // non-positive attributes are not allowed in MFEM
  if (phys_domain <= 0)
  {
  - MFEM_ABORT("Non-positive element attribute in Gmsh mesh!");
  + MFEM_ABORT("Non-positive element attribute in Gmsh mesh!\n"
  + "Use \"Physical Curve\", \"Physical Surface\","
  + " or \"Physical Volume\" to set attributes "
  + "for all curves, surfaces, or volumes in your "
  + "Gmsh geometry.");
  }
  // initialize the mesh element
  