Add a switch to the nexus program to raise exceptions on errors

[jfilby]

Right now the code sometimes shows error messages and quits, and sometimes raises an exception with the same information and a stacktrace.

By default only the stdout errors should be shown. When a -e or --exceptions-on-errors switch is passed to the nexus program, then an exception should be raised instead of quit(1). This displays a stacktrace that could be useful when debugging issues.