Enhanced error message when st2_prep_release is run twice

[nmaludy]

When working on the 3.3 release i accidentally ran the st2_prep_release action too soon (going to fix this by editing the Release process Wiki). Coming back to this a week or so later, i thought about running this action again and came across a bad error message that told me what was wrong, but didn't tell me what i should be doing instead.

I've edited the error message with some helpful guidance to the next person performing a release, so hopefully they won't stub their toe like i did.

[arm4b]

Makes sense and helps future generations of Stormers hitting the same issue 👍

[blag]

That's a looooong single line, but it's a great error message now!

[CLAassistant]

All committers have signed the CLA.