Require TryFrom::Error to impl Debug

[leighmcculloch]

As example to https://github.com/stellar/rs-stellar-contract-sdk/pull/149/files#r914120998

Close #161

[jonjove]

Related to #161

[leighmcculloch]

Note that in the future, once associated type bounds (rust-lang/rust#52662) are stabilized, we'd probably be able to omit the need for the constraint on every function in the SDK. We'd be able to change the definition of IntoTryFromVal so that it bounds the TryFrom::Error to Debug with the following change. Likely a similar change in the env crate would help us out up and down the stack.

- pub trait IntoTryFromVal: IntoVal<Env, RawVal> + TryFromVal<Env, RawVal> {
+ pub trait IntoTryFromVal: IntoVal<Env, RawVal> + TryFromVal<Env, RawVal, Error: Debug> {