Error message is slightly misleading if ZIM file can not be open

[kelson42]

$ zimcheck toto.zim
...
Error opening as a split file: toto.zim

Here it should have error like:

Error opening ZIM file: toto.zim

AND

$ zimcheck toto.zimaa
...
Error opening as a split file: toto.zim

Here it should have error like:

Error opening ZIM file: toto.zimaa