modified error message

After talking with Frantisek and Jakub, I modified his error message. Kind of test to see if I can branch properly!
This does not seem right:

Current message example: 
```
message-body asset is expected to be a pre-formatted code block, every of its line indented by exactly 8 spaces or 2 tabs
```

Proposed change:

```
message-body asset this entry is expected to be a pre-formatted code block, with each line indented by exactly 8 spaces or 2 tabs
```

Also I would like to use same terminology as in API Blueprint spec – "entry" is never used in the context of a blueprint "section".