Consider augmenting _Scope to ensure we clear errors after each operation

[jonasfj]

We wrap all (or almost all) FFI code in _Scope, thus, it might make sense to review all usage of _Scope to see if we can augment _Scope such that that clear errors after an operation is done.

We should be handling errors all the places where they can happen, but as added safety it might be wise to clear errors after each operation and throw an error, if BoringCrypto has an error on the stack that we didn't expect to find there.

[jonasfj]

See also: https://github.com/google/webcrypto.dart/issues/63