Implement TryFrom<&Error> for ScVal

[brson]

What

impl TryFrom<&Error> for ScVal

Why

Error cannot be stored in #[contracttype]s.

This impl is required for types to be stored in #[contracttype]. Without it the compiler produces errors like

error[E0277]: the trait bound `stellar_xdr::ScVal: From<&soroban_env_host::Error>` is not satisfied
    --> soroban-sdk/src/arbitrary.rs:1027:9
     |
1027 |         #[contracttype]
     |         ^^^^^^^^^^^^^^^ the trait `From<&soroban_env_host::Error>` is not implemented for `stellar_xdr::
ScVal`
     |
     = note: required for `&soroban_env_host::Error` to implement `Into<stellar_xdr::ScVal>`
     = note: required for `stellar_xdr::ScVal` to implement `TryFrom<&soroban_env_host::Error>`
     = note: required for `&soroban_env_host::Error` to implement `TryInto<stellar_xdr::ScVal>`
     = note: this error originates in the attribute macro `contracttype` (in Nightly builds, run with -Z macro-
backtrace for more info)
help: consider dereferencing here
     |
1027 |         (*#[contracttype])
     |         ++               +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `soroban-sdk` (lib test) due to previous error
warning: build failed, waiting for other jobs to finish...

Known limitations

na