BlindTransaction() and VerifyAmounts() need detailed error return codes

The functions `BlindTransaction()` and `VerifyAmounts()` each have several failure paths, but only use -1 and false to indicate to the caller that an operation failed. This makes it difficult to report helpful error messages to the user.

The error paths could return an enum indicating the reason for failure, and the callers could then surface error strings to the user.

`BlindTransaction()`: https://github.com/ElementsProject/elements/blob/master/src/blind.cpp#L286
`VerifyAmounts()`: https://github.com/ElementsProject/elements/blob/master/src/confidential_validation.cpp#L134