Error message can be misleading if spring.config.import fails to resolve

See #35683.

Given:
```properties
spring.config.import = exmple:some-path
```

We'll get the error:

```
Caused by: java.lang.IllegalStateException: File extension is not known to any PropertySourceLoader. If the location is meant to reference a directory, it must end in '/' or File.separator
```

Which is somewhat misleading.