fix: expose Err() method to check for configuration errors

[Yanhu007]

Fixes #89

Problem

New() silently stores configuration errors (missing hash key, invalid block cipher key) in a private err field. There is no way for callers to check if New() succeeded — they only discover the problem when Encode()/Decode() later returns an error.

s := securecookie.New(nil, nil) // no error returned!
_, err := s.Encode("session", value) // fails here with cryptic error

Fix

Add a public Err() method:

func (s *SecureCookie) Err() error

This lets callers check for misconfiguration immediately:

s := securecookie.New(hashKey, blockKey)
if err := s.Err(); err != nil {
    log.Fatalf("securecookie setup failed: %v", err)
}

This is backward-compatible — no existing API signatures change. All existing tests pass.