Improve logging when loading libclang fails

- Add exception to error message in debug mode.
- Add check for non-existent string
- Squash strings together, since Vim will print multi-line strings but
  only displays the first string if multiple strings are emitted, using:

```
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Oct 19 2013 20:22:55)
MacOS X (unix) version
Included patches: 1-52
Compiled by Homebrew
```

It seems I'm not the only one to have issues on Mavericks:

http://stackoverflow.com/a/20645984/78845