Remove all system.exit(0)

[Benjamin Cordes]

all system.exists(0) should be handled on that level so critical errors bubble up to highest level and then system exits. maybe introduce a custom Exception for this (NuBotCriticalOperationError, NuBotConfigError, etc).

"There are some places inside the execution threads that are ok others, like options loading, that are not"

see also Utils.exitcritical (?)