Shouldn't crash if tag already exists

[FrancescoCioria]

description

if you use smooth-release to release vX.Y.Z but you already have a tag X.Y.Z, smooth-release crashes

how to reproduce

• {optional: describe steps to reproduce defect}

specs

if there is already a tag for the version the user wants to release

• throw an error and abort before doing anything destructive (so before updating package.json)

misc

{optional: other useful info}