Use value.inspect to prevent interpoliation errors

This way of interpolation will fail, for example when the invalid value is an empty array.

Example:

```
value = []
"does not accept %p as value" % value
```
Merge pull request #8 from fabrik42/develop

Bug fix: Prevent interpolation errors when constructing error messages
Updates version: v1.5.0 -> v1.5.1
Release v1.5.1