Add detail error code for internal errors

[plateaukao]

Summary

Add errorCode for LineApiError class, so that when internal error happens, it's easier to identify what kind of error it is by checking the errorCode enum type.

Currently supported types are

    public enum ErrorCode {
        LOGIN_ACTIVITY_NOT_FOUND, // when line-sdk can't find Activity that supports login intent.
        HTTP_RESPONSE_PARSE_ERROR, // when line-sdk can't parse http response from line login server.
        NOT_DEFINED, // when error code is not defined.
    }

[JerryMig]

LGTM