vtk error handling is not very helpful

python/damask/_vtk.py

@@ -1,3 +1,4 @@
+import os
 import multiprocessing as mp
 from pathlib import Path
 
@@ -165,6 +166,9 @@ class VTK:
             else:
                 raise TypeError(f'Unknown file extension {ext}')
 
+            if not os.path.isfile(fname):
+                raise FileNotFoundError(f'No such file: {fname}')
+
             reader.SetFileName(str(fname))
             reader.Update()
             vtk_data = reader.GetOutput()