More verbose error for whole resource move failure

When running state mv with a resource source, but the destination
fails, provide a hint that the source is a resource (not an instance)
in case the user means to address it this way

command/state_mv.go

@@ -184,7 +184,7 @@ func (c *StateMvCommand) Run(args []string) int {
 				diags = diags.Append(tfdiags.Sourceless(
 					tfdiags.Error,
 					msgInvalidTarget,
-					fmt.Sprintf("Cannot move %s to %s: the target must also be a whole resource.", addrFrom, destAddr),
+					fmt.Sprintf("Cannot move %s to %s: the source is a whole resource (not a resource instance) so the target must also be a whole resource.", addrFrom, destAddr),
 				))
 				c.showDiagnostics(diags)
 				return 1

command/state_mv_test.go

@@ -293,8 +293,9 @@ func TestStateMv_resourceToInstanceErr(t *testing.T) {
 	expectedErr := `
 Error: Invalid target address
 
-Cannot move test_instance.foo to test_instance.bar[0]: the target must also be
-a whole resource.
+Cannot move test_instance.foo to test_instance.bar[0]: the source is a whole
+resource (not a resource instance) so the target must also be a whole
+resource.
 
 `
 	errOutput := ui.ErrorWriter.String()