Fix ExactSolution w/ coarse DistMesh + fine mesh

This isn't a good fix, because it precludes users carefully setting up
ghosting manually between meshes... but since we're already serializing
the solution here, we might as well serialize the mesh too.

This fixes --enable-distmesh runs of miscellaneous_ex10 on 3+
processors.

Author: roystgnr

src/error_estimation/exact_solution.C

@@ -24,6 +24,7 @@
 #include "libmesh/function_base.h"
 #include "libmesh/mesh_base.h"
 #include "libmesh/mesh_function.h"
+#include "libmesh/mesh_serializer.h"
 #include "libmesh/numeric_vector.h"
 #include "libmesh/parallel.h"
 #include "libmesh/quadrature.h"
@@ -475,9 +476,14 @@ void ExactSolution::_compute_error(std::string_view sys_name,
   const unsigned int var_component =
     computed_system.variable_scalar_number(var, 0);
 
-  // Prepare a global solution and a MeshFunction of the coarse system if we need one
+  // Prepare a global solution, a serialized mesh, and a MeshFunction
+  // of the coarse system, if we need them for fine-system integration
   std::unique_ptr<MeshFunction> coarse_values;
-  std::unique_ptr<NumericVector<Number>> comparison_soln = NumericVector<Number>::build(_equation_systems.comm());
+  std::unique_ptr<NumericVector<Number>> comparison_soln =
+    NumericVector<Number>::build(_equation_systems.comm());
+  MeshSerializer
+    serial(const_cast<MeshBase&>(_equation_systems.get_mesh()),
+           _equation_systems_fine);
   if (_equation_systems_fine)
     {
       const System & comparison_system