client/keys: --algo flag silently falls back to default if user supplied algo is not supported

[alessio]

	algo, err := keyring.NewSigningAlgoFromString(viper.GetString(flagKeyAlgo))
	if err != nil {
		algo = hd.Secp256k1
	}

Proposed solution, as discussed with @fedekunze, is to add a function to Keyring interface through which the caller can retrieve the list of algos supported by the keyring itself.

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