Pred error messages

This incorporates feedback from Jason Wolfe.

Before:

```
(schema/validate Country "asdf")
ExceptionInfo Value does not match schema: (not (#<core$countries_contains_QMARK_ schema_contrib.core$countries_contains_QMARK_@359d924a> "asdf"))  schema.core/validate (core.clj:165)
```

After:

```
(schema/validate Country "asdf")
ExceptionInfo Value does not match schema: (not (Country "asdf"))  schema.core/validate (core.clj:165)
```