Add information to FVFluxBC error on faces it should not be triggered on

Add test for FVFluxBCs to capture this error
Add test for FVNeumannBC for regular use
refs idaholab#16882

framework/src/fvbcs/FVFluxBC.C

@@ -63,8 +63,13 @@ FVFluxBC::computeResidual(const FaceInfo & fi)
     prepareVectorTag(_assembly, _var.number());
   else if (ft == FaceInfo::VarFaceNeighbors::NEIGHBOR)
     prepareVectorTagNeighbor(_assembly, _var.number());
+  else if (ft == FaceInfo::VarFaceNeighbors::BOTH)
+    mooseError("A FVFluxBC is being triggered on an internal face with centroid: ",
+               fi.faceCentroid());
   else
-    mooseError("should never get here");
+    mooseError("A FVFluxBC is being triggered on a face which does not connect to a block ",
+               "with the relevant finite volume variable. Its centroid: ",
+               fi.faceCentroid());
 
   _local_re(0) = r;
   accumulateTaggedLocalResidual();
@@ -160,8 +165,13 @@ FVFluxBC::computeJacobian(const FaceInfo & fi)
       computeJacobian(Moose::ElementElement, residual);
     else if (ft == FaceInfo::VarFaceNeighbors::NEIGHBOR)
       computeJacobian(Moose::NeighborNeighbor, residual);
+    else if (ft == FaceInfo::VarFaceNeighbors::BOTH)
+      mooseError("A FVFluxBC is being triggered on an internal face with centroid: ",
+                 fi.faceCentroid());
     else
-      mooseError("should never get here");
+      mooseError("A FVFluxBC is being triggered on a face which does not connect to a block ",
+                 "with the relevant finite volume variable. Its centroid: ",
+                 fi.faceCentroid());
   };
 
   _assembly.processDerivatives(r, _var.dofIndices()[0], _matrix_tags, local_functor);

test/tests/fvbcs/fv_neumannbc/fv_neumannbc.i

@@ -0,0 +1,73 @@
+[Mesh]
+  [mesh]
+    type = CartesianMeshGenerator
+    dim = 2
+    dx = '1 1'
+    dy = '1'
+    ix = '5 5'
+    iy = '5'
+    subdomain_id = '1 1'
+  []
+  [internal_sideset]
+    type = ParsedGenerateSideset
+    combinatorial_geometry = 'x<1.01 & x>0.99'
+    included_subdomain_ids = 1
+    new_sideset_name = 'center'
+    input = 'mesh'
+  []
+[]
+
+[Variables]
+  [u]
+    family = MONOMIAL
+    order = CONSTANT
+    fv = true
+    block = 1
+  []
+[]
+
+[FVKernels]
+  [diff]
+    type = FVDiffusion
+    variable = u
+    coeff = 1
+  []
+[]
+
+[FVBCs]
+  inactive = 'center'
+  [left]
+    type = FVDirichletBC
+    variable = u
+    boundary = left
+    value = 1
+  []
+  [right]
+    type = FVNeumannBC
+    variable = u
+    boundary = right
+    value = 4
+  []
+  # Internal center sideset, should cause erroring out
+  [center]
+    type = FVNeumannBC
+    variable = u
+    boundary = center
+    value = 0
+  []
+[]
+
+[Problem]
+  kernel_coverage_check = off
+[]
+
+[Executioner]
+  type = Steady
+  solve_type = 'Newton'
+  petsc_options_iname = '-pc_type'
+  petsc_options_value = 'lu'
+[]
+
+[Outputs]
+  exodus = true
+[]

test/tests/fvbcs/fv_neumannbc/tests

@@ -0,0 +1,28 @@
+[Tests]
+  [fv_neumann]
+    design = 'FVNeumannBC.md'
+    issues = '#16477'
+    type = 'Exodiff'
+    input = 'fv_neumannbc.i'
+    exodiff = 'fv_neumannbc_out.e'
+    requirement = 'The system shall run a simple 1D diffusion problem with a Neumann BC value.'
+  []
+  [fvbcs_internal]
+    design = 'FVNeumannBC.md'
+    issues = '#16882'
+    type = RunException
+    input = 'fv_neumannbc.i'
+    cli_args = "FVBCs/inactive=''"
+    expect_err = 'A FVFluxBC is being triggered on an internal face with centroid'
+    requirement = 'The system shall error out if FVFluxBC, in this case a FVNeumannBC, are used internally.'
+  []
+  [fvbcs_disconnected_from_variable]
+    design = 'FVNeumannBC.md'
+    issues = '#16882'
+    type = RunException
+    input = 'fv_neumannbc.i'
+    expect_err = 'A FVFluxBC is being triggered on a face which does not connect to a block with the relevant finite volume variable.'
+    cli_args = "Mesh/mesh/subdomain_id='1 2'"
+    requirement = 'The system shall error out if FVFluxBC are used on a mesh face that is not connected to an element with the corresponding finite volume variable.'
+  []
+[]

test/tests/transfers/multiapp_userobject_transfer/tests

@@ -53,7 +53,7 @@
     input = 'restricted_elem_master.i'
     exodiff = 'restricted_elem_master_out.e restricted_elem_master_out_ch00.e restricted_elem_master_out_ch10.e'
 
-    requirement = "The system shall transfer user-defined spatial data into block and boundary restricted elemntal variables."
+    requirement = "The system shall transfer user-defined spatial data into block and boundary restricted elemental variables."
     design = "MultiAppUserObjectTransfer.md"
     issues = '#16241'
   []