GenerateRandomKey: do not swallow errors

The requirement to check for nil returns is so unexpected that even
other Gorilla libraries get it wrong:

<https://github.com/gorilla/sessions/blob/3eed1c4ffcde6f23b6f88068c63c1ef6190df331/store.go#L225>

Since a malfunction of the system random number generator is pretty
unrecoverable for most security-sensitive applications, I consider it
fine to use a panic here. Most callers will have no better option than
to just die anyway. If callers need a more specific behavior, they can
implement these three lines of code themselves with application-specific
error handling.

securecookie.go

@@ -510,12 +510,12 @@ func decode(value []byte) ([]byte, error) {
 // persisted. New keys will be created when the application is restarted, and
 // previously issued cookies will not be able to be decoded.
 //
-// Callers should explicitly check for the possibility of a nil return, treat
-// it as a failure of the system random number generator, and not continue.
+// Panics if the system random number generator cannot come up with the requested
+// amount of randomness.
 func GenerateRandomKey(length int) []byte {
 	k := make([]byte, length)
 	if _, err := io.ReadFull(rand.Reader, k); err != nil {
-		return nil
+		panic(fmt.Sprintf("could not generate %d bytes of randomness: %s", length, err.Error()))
 	}
 	return k
 }