Issues with config import

An `import` statement in config files is currently not working if the path contains less than 3 slashes, e.g.
```
import /foo/file
```
Also, any issue with invalid file paths is masked. The following import statements do not display any error message:
```
import /path/to/not/readable
import /path/to/non/existing
import does + not - make / sense
```

Start by tracing how config-file import statements are parsed and how invalid paths are handled, then reproduce the examples for short paths, unreadable files, missing files, and malformed syntax. Done means imports with fewer than three slashes work and each invalid import reports an appropriate error instead of being silently ignored.

Written by the indexing model from the issue text.