Merge branch 'doc/src_file_error_message' into 'master'

adds error message

See merge request !144

TurTLE/DNS.py

@@ -1007,6 +1007,12 @@ class DNS(_code):
                             f0.close()
                             src_file = check_file
                             break
+                if type(src_file) == type(None):
+                    print('could not find checkpoint file')
+                    print('source_checkpoints = ', source_checkpoints)
+                    print('checkpoint_field = ', checkpoint_field)
+                    print('iteration = ', opt.src_iteration)
+                    exit(0)
                 self.copy_complex_field(
                         src_file,
                         checkpoint_field + '/complex/{0}'.format(opt.src_iteration),