More helpful exceptions

[gjohnson]

Mainly user error, but exceptions (when starting up) are a bit un-helpful. Here is an example of a invalid data directory on OSX.

[INFO] 2012-Sep-22 17:24:47.901668: darner: queue server
[INFO] 2012-Sep-22 17:24:47.902440: build: Aug 30 2012 (17:15:58) v0.1.4 (c) Wavii, Inc.
[INFO] 2012-Sep-22 17:24:47.902481: listening on port: 22133
[INFO] 2012-Sep-22 17:24:47.902515: data dir: /tmp/
[INFO] 2012-Sep-22 17:24:47.902544: debug logging is turned ON
[INFO] 2012-Sep-22 17:24:47.902574: starting up
libc++abi.dylib: terminate called throwing an exception
Abort trap: 6

[erikfrey]

I agree - some improvements like this will go into the next batch of changes.