Typos in network name should be identified as such

[iameli-streams]

I misspelled arbitrum-one-mainnet as arbirum-one-mainnet and got a cryptic error back:

▶ ./bin/livepeer -broadcaster -network arbirum-one-mainnet -ethUrl https://arb1.arbitrum.io/rpc -ethKeystorePath ~/.ethereum/keystore
*-----------------*---------------------------------*
| Network         | arbirum-one-mainnet             |
| Broadcaster     | true                            |
| EthKeystorePath | /home/iameli/.ethereum/keystore |
| EthUrl          | https://arb1.arbitrum.io/rpc    |
*-----------------*---------------------------------*
I0314 16:22:56.737971   43218 starter.go:391] ***Livepeer is running on the arbirum-one-mainnet network***
I0314 16:22:56.889897   43218 accountmanager.go:80] Using Ethereum account: 0x648D909D6Ebbbf016f34f9eEBE608Ad62aa225C9
I0314 16:22:57.388705   43218 accountmanager.go:103] Please enter the passphrase to unlock Ethereum account 0x648D909D6Ebbbf016f34f9eEBE608Ad62aa225C9
Passphrase: 
I0314 16:23:02.569597   43218 accountmanager.go:114] Unlocked ETH account: 0x648D909D6Ebbbf016f34f9eEBE608Ad62aa225C9
E0314 16:23:02.717474   43218 client.go:199] Error getting LivepeerToken address: no contract code at given address
E0314 16:23:02.717521   43218 starter.go:635] Failed to set gas info on Livepeer Ethereum Client: no contract code at given address

We should presumably just exit early if we've never heard of the network you're using; there's not going to be deployed contracts there and stuff, yeah?