Fix crash without extension on Rococo

[kwingram25]

Closes #305

• Sets default accountId value correctly without crashing in event that Polkadot extension is not found
• Display blocking error message with instructions to install extension, in case no accounts found on a testnet

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[statictype]

maybe we should add some info about how to drip funds? https://github.com/paritytech/cumulus/blob/master/parachains/runtimes/contracts/contracts-rococo/README.md

[statictype]

added one last comment, if there is nothing else you'd like to do, we can merge it

[kwingram25]

Just a couple things I found. Thanks for the improvements!