Returns true iff there were no "internal" errors. In other words, errors that has nothing to do with structure checking. If an "internal" error occurred, the appropriate Python error will be set and the caller can propage it directly to the user. Both `error_msg` and `is_type_error` must be non-null. `error_msg` must be empty. Leaves `error_msg` empty if structures matched. Else, fills `error_msg
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