Convert chain RuleError to TxRuleError in mempool.

When a transaction is being checked for acceptance into the transation
memory pool, it makes use of a chain function to ensure the invariant rules
for what transactions are allowed into a block are not violated.  That
function returns a btcchain.RuleError if rules are violated.  However,
since this code path is tailored to free-standing transactions, the error
needs to be converted to a TxRuleError so the caller can properly detect
the transaction as a rejected transaction instead of treating it like an
real error.