Typos in network name should be identified as such

Nobody has claimed this yet.

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?

Start by reading the network startup flow in starter.go, especially around lines 391 and 635, then inspect accountmanager.go and client.go where the cryptic contract error appears. Trace how the -network value is accepted and determine where unknown networks should be rejected; done means a misspelled network exits early with a clear error instead of attempting account setup and contract access.

Written by the indexing model from the issue text.