Introduce better handling of codestyle errors

Currently the build completely stops when a section of code fails the Codestyle check, even though it is perfectly compilable.
This pull request changes:
```
[...]
Checking codestyle
admin/angel/Core.c:1  missing header
/* vim: set expandtab ts=4 sw=4: */
/* vim: set expandtab ts=9999 sw=asdasdasdasdqwde: */

Failed to build cjdns.
Total build time: 108000ms.
```
into:
```
[...]
Checking codestyle
admin/angel/Core.c:1  missing header
/* vim: set expandtab ts=4 sw=4: */
/* vim: set expandtab ts=9999 sw=asdasdasdasdqwde: */

Test 593ms
Pack 16ms
Get mtimes 9ms
Save State 96ms
Build compiled successfully but there was some errors.
Total build time: 10000ms.
```