[CAD-1462] Clarify insert ticker command name and report error if ticker already exists

[ksaric]

https://jira.iohk.io/browse/CAD-1462

Based on #45

Improvement reported by @dorin100
Example:

$> SMASHPGPASSFILE=config/pgpass stack run smash-exe -- reserve-ticker-name --tickerName "EARTH" --poolhash "0bbae0287d0ee96cc63b07cc18d25483d9010282ac726c35f450ea8622036e87"
Reserving ticker name!
Adding reserved ticker 'EARTH' with hash: PoolMetadataHash {getPoolMetadataHash = "0bbae0287d0ee96cc63b07cc18d25483d9010282ac726c35f450ea8622036e87"}
Ticker name inserted into the database reserved!

$> SMASHPGPASSFILE=config/pgpass stack run smash-exe -- reserve-ticker-name --tickerName "EARTH" --poolhash "0bbae0287d0ee96cc63b07cc18d25483d9010282ac726c35f450ea8622036e87"
Reserving ticker name!
Adding reserved ticker 'EARTH' with hash: PoolMetadataHash {getPoolMetadataHash = "0bbae0287d0ee96cc63b07cc18d25483d9010282ac726c35f450ea8622036e87"}
Reserved ticker name not inserted! Ticker 'EARTH' has already been inserted.