Improve an error message.

In particular, say which file could not be opened and what the error code is.

diff --git a/source/base/data_out_base.cc b/source/base/data_out_base.cc
index 8e5a78bbd70e1fd700d48ee56003347700a14d60..9e14e4aa7f2f1e96771e86b06e0717716ade94c7 100644 (file)
--- a/source/base/data_out_base.cc
+++ b/source/base/data_out_base.cc
@@ -6153,7 +6153,12 @@ void DataOutInterface<dim,spacedim>::write_vtu_in_parallel (const char *filename
   MPI_File fh;
   err = MPI_File_open(comm, const_cast<char *>(filename),
                       MPI_MODE_CREATE | MPI_MODE_WRONLY, info, &fh);
-  AssertThrow(err==0, ExcMessage("Unable to open file with MPI_File_open!"));
+  AssertThrow(err==0,
+              ExcMessage("Unable to open file <"
+                         + std::string(filename) +
+                         "> with MPI_File_open. The error code "
+                         "returned was "
+                         + Utilities::to_string(err) + "."));
 
 
   MPI_File_set_size(fh, 0); // delete the file contents