Improvement to logging when loading libclang fails

Problem
-------

libclang failed to load on OS X Mavericks with XCode 5.0.1 and the existing logging proved insufficient to track this down.

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

Changes
-------

- 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
```
Merge pull request #357 from johnsyweb/better_logging

Improvement to logging when loading libclang fails