Following changes were made:
1) Catch and signal error for duplicate string destinations.
2) Catch and signal error for invalid value type.
3) Catch and signal error for string destination not found in tx outputs.
4) Improved 'InterpretSubtractFeeFromOutputInstructions()' code organization.
5) Added test coverage for all possible error failures.
Also, fixed two PEP 8 warnings at the 'wallet_sendmany.py' file:
- PEP 8: E302 expected 2 blank lines, found 1 at the SendmanyTest class declaration.
- PEP 8: E303 too many blank lines (2) at skip_test_if_missing_module() and set_test_params()
If the serialized transaction passed to `fundrawtransaction` contains
duplicates, they will be deserialized and added to the transaction. Add
a test to ensure this behavior is not changed during the refactor.
A user can pass any number of duplicated and unrelated addresses as an
SFFO argument to `sendmany` and the RPC will not throw an error (note,
all the rest of the RPCs which take SFFO as an argument will error if
the user passes duplicates or specifies outputs not present in the
transaction). Add a test to ensure this behavior is not changed during
the refactor.