Improve an error message.

diff --git a/include/deal.II/numerics/derivative_approximation.h b/include/deal.II/numerics/derivative_approximation.h
index be3854dd8cc759be08f1b78f00f84d415e4852ab..bbe1450348c43112a3071022266e65343822c871 100644 (file)
--- a/include/deal.II/numerics/derivative_approximation.h
+++ b/include/deal.II/numerics/derivative_approximation.h
@@ -307,11 +307,16 @@ namespace DerivativeApproximation
    * Exception
    */
   DeclExceptionMsg(ExcInsufficientDirections,
-                   "We have encountered a cell on which the number of linearly "
+                   "While computing a finite difference approximation to "
+                   "derivatives, the algorithm encountered a cell on which "
+                   "the number of linearly "
                    "independent directions that span the matrix Y (discussed "
                    "in the documentation of the DerivativeApproximation "
                    "class) is not equal to dim. The matrix Y then is "
-                   "rank deficient and can not be inverted.");
+                   "rank deficient and can not be inverted. A common reason "
+                   "why this might be happening is if a cell has neither "
+                   "left/right (or up/down, or front/back) neighbors, for "
+                   "example because the mesh is too coarse.");
 } // namespace DerivativeApproximation