Report details about fork() failure in logs.

When parsing a large codebase dot failed to start. Adding this line helped narrow down the issue. Increasing my VirtualBox VM RAM size worked around the issue. A proper fix would be to spawn separate processes without using fork() so they don't start with the giant address space used by the current process.
Merge pull request #65 from pianofab/patch-1

Report details about fork() failure in logs.