Remove public From<String> implementations from FFI error wrappers

[spacebear21]

Following up on #737, there are three remaining FFI error types that implement From<String>: PjParseError, PjNotSupported and OhttpError.

Ideally these types should just wrap the inner payjoin error instead of using a custom msg: String inner value. If that's not possible due to complex inner types, at the very least we should use map_err instead of exposing From<String> in the public API.

[arminsabouri]

Assigned to @chavic

[spacebear21]

Closed by #1127 . I'd like to follow up by fixing the underlying rust-payjoin error types so that we don't need to stringify the contents for FFI and can just wrap around the error with #[error(transparent)].