Expose MySqlErrorCode from MySqlException

[bgrainger]

Most MySqlException objects have a corresponding MySqlErrorCode. This is exposed through the public int Number { get; } property, which requires a (non-obvious) cast.

There's a virtual int ErrorCode { get; } property inherited from ExternalException, which appears to always return E_FAIL: https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/ExternalException.cs

Since this isn't useful, and is already accessible through Exception.HResult, define public new MySqlErrorCode ErrorCode { get; } and return the error code that way.

(Connector/NET adds a confusingly-named Code property that's only defined for the X Protocol: https://stackoverflow.com/a/60293352/23633.)