Return a more informative error message when trying to spend coinbase; select non-coinbase inputs when sending to a transparent output if needed
For #1373 and #1519
Code change:
- Extra parameter added to AvailableCoins to include or exclude Coinbase coins. Default value of parameter is 'true' as current behaviour is to include Coinbase coins.
- SelectCoins, used for sending taddr->taddr, will now exclude Coinbase coins.
Unit test:
Tried to write a test to focus on the extra parameter added to AvailableCoins but could not.
Empirical testing on Testnet:
Current behaviour is that upstream RPC commands sendfrom and sendtoaddress try to spend coinbase coins returned by AvailableCoins. So the user will see:
```
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Error: The transaction was rejected! This might happen if some of the coins in your wallet were already spent, such as if you used a copy of wallet.dat and coins were spent in the copy but not marked as spent here."}
./zcash-cli sendfrom "" mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Error: The transaction was rejected! This might happen if some of the coins in your wallet were already spent, such as if you used a copy of wallet.dat and coins were spent in the copy but not marked as spent here."}
```
After fix is applied:
```
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Coinbase funds can only be sent to a zaddr"}
```
When non-coinbase UTXOs exist, they will now be selected and used:
```
./zcash-cli z_sendmany tnPJZHeVxegCg91utaquBRPEDBGjozfz9iLDHt7zvphFbZdspNgkTVLCGjDcadQBKNyUwKs8pNjDXuEZKrE1aNLpFwHgz4t '[{"address":"mx5fTRhLZwbYE7ZqhAPueZgQGSnwTbdvKU", "amount":0.01}]'
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
9818e543ac2f689d4ce8b52087607d73fecd771d45d316a1d9db092f0485aff2
./zcash-cli sendfrom "" mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
899f2894823f51f15fc73b5e0871ac943edbe0ff88e1635f86906087b72caf30
```
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