Provide error variants for transaction failures

[geigerzaehler]

Submitting a transaction to a node for inclusion in block can fail for a multitude of reasons. These reasons should be exposed in the client Error enum.

Sources of transaction errors is the TransactionStatus stream and the response from rpc.watch_extrinsic. We should handle all TransactionStatuss apropriately instead of returning InvalidTransactionStatus.

We should at least provide tests for the invalid transaction case. If possible we should also test more transaction errors. The end-to-end tests already include some tests for transaction errors.