Validate denom on `cw721-piggy-bank` instantiation

[JakeHartnell]

See:

code-and-chill/contracts/cw721-piggy-bank/src/contract.rs

Line 30 in 0c34581

// TODO Validate denoms are formated correctly

[JakeHartnell]

Be sure to write a test for this. Look at other contracts for inspiration.