Throw a standard Error when transaction is not found

[jcramer]

When the validator is initialized we need to check that the user-supplied method for fetching transactions (i.e., getRawTransaction) will throw when txids that do not exist are requested. This can be done by simply calling getRawTransaction during ValidatorType1 instantiation or first call to isSlpTxidValid.

• use some impossible txid like 0000...0000
• a short timeout should be used in case the user-suppled method hangs.
• regardless of what the user's method throws, we should replace that by throwing the a consistent message of "Transaction data for the provided txid not found"