[devex] use anyhow crate for Result instead of failure

[sblackshear]

FastPay was written at a time when the failure crate was in vogue, but it is now deprecated. anyhow is a vastly superior alternative that should facilitate creating errors and interacting with other crates that expose errors (which will invariably use anyhow). We should probably make this switch sooner rather than later to make the refactoring less painful...