Export common errors from decode methods.

[hanlhe]

This PR is a minor improvement for error handling. It defines and exports the common errors in the protocol decode methods.

• Usually, the same error would occur repeatedly, like fmt.Errorf("invalid netflow version (%d)", h.Version). With predefined errors, it would be easier for the user of the library to aggregate the error stats.
• The static errors (created by errors.New) are also predefined and reused, instead of creating new one on the fly.

[hanlhe]

Hi @mehrdadrad, could you please review the PR and see if the changes make sense to you? Thank you!

[hanlhe]

I will reopen this later.