don't crash if ~/Library/Logs/DiagnosticReports does not exist.

This was pretty bad - xctool would actually crash if this directory
didn't exist (meaning you had no crash reports on your system).

Fixes issue #11

Tested with...

```
./xctool.sh -workspace
xctool/xctool-tests/TestData/TestWorkspace-Library/TestWorkspace-Library.xcworkspace
-scheme TestProject-Library -configuration Debug -arch i386 -sdk
iphonesimulator test -only TestProject-LibraryTests
```
Merge pull request #14 from facebook/fpotter/dont-crash-when-there-are-no-crash-reports

don't crash if ~/Library/Logs/DiagnosticReports does not exist.