example.cpp: return 0, or exit(1)

For example to be usable as a simple test, it needs to return
a proper.
example.cpp: check params and show usage

example.cpp needs to validate the parameters passed to it and
show a usage message if the parameters are invalid.
Merge branch 'example-fixes' of https://github.com/reubenhwk/nanodbc into reubenhwk-example-fixes
Some small style related changes.