[Client] Added misconsidered error handling

[hyunsooda]

Proposed changes

The client function TransactionInBlock() had not correctly handled the internal function call error. The expected error should be the block does not exist or something others, but it makes a runtime panic (at L241). This PR added an early exit when the return value of the internal function call is found as an error.

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• Bugfix
• New feature or enhancement
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Further comments

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