Return non-zero exit codes when something goes wrong (WIP)

[MinasMazar]

This close #52

Some references around the web (like this) tells that is a standard to return a value of 0 when the command performed successfully and a value different form 0 otherwise.

So this is the exit_status table for our project:

CASE	EXIT CODE
all commands successfully terminated	0
invalid pulsar parameters	1
other exceptions	1

IMPORTANT

I think this PR will never solve the related issue: there is an unresolvable conflict between Kernel::exit and simplecov gem. Maybe the cause is related to at_exit callback used by simplecov. I noticed many unpredictable weirds behaviors. I tried to keep tests green and full coverage, with many tricks and workarounds but I lost the challenge.

NB. The statement with Kernel::exit method is not covered by default by ruby built-in coverage mechanism, so in any case we'll get a 100% coverage result when using this method.

[kennyadsl]

maybe this can help: https://bibwild.wordpress.com/2017/09/15/consider-ttycommand-for-all-your-external-processshell-out-needs-in-ruby/ ?