Move most safety checks into the library

[ricmoo]

Currently the bin/ethers-ens performs a large number of safety checks, for example:

• Make sure a bid exists on the blockchain before attempting to reveal
• Make sure a bid does not exist on the blockchain before bidding
• Make sure an auction is owned before allowing finalize
• Make sure the start date is past before allowing starting an auction
• Make sure the command, in general, makes sense for the current state

These checks should be moved into the library (index.js) so all callers benefit from them.

This will also need to have more machine-readable errors passed back (via the Promise), and have those machine-readable errors converted back into human-readable.

[ricmoo]

This was never complete. In the future this may become an ancillary package, but is not a priority right now...