Config after incorrect import is silently ignored

Configuration values after invalid imports are silently ignored.

The `timewarrior.cfg`

```config
import /usr/local/share/doc/timew/holidays/holidays.en-US # not existing file
reports.summary.ids = yes
reports.summary.annotations = yes
```

gives

```sh
$ timew config
<...>
summary:
holidays = yes
<...>
```

while

```config
reports.summary.ids = yes
reports.summary.annotations = yes
import /usr/local/share/doc/timew/holidays/holidays.en-US # not existing file
```

gives

```sh
$ timew config
<...>
summary:
annotations = yes
holidays = yes
ids = yes
<...>
```

Inboth cases no error message is printed.

EDIT: adjusted after I realized that the filepath was wrong.

Start by reproducing both configurations with the `timew config` command and compare how the invalid import affects subsequent values. Trace the configuration import handling from that entry point; done means invalid imports produce an error and do not silently discard later configuration values.

Written by the indexing model from the issue text.