docs: document valid mnemonics and print the guidance in error message

[odeke-em]

Beginners trying to get to speed with Ethermint will likely find themselves stuck trying to guess what a valid mnemonic is while trying to run a node https://ethermint.dev/quickstart/run_node.html yet there is zero guidance for what a valid mnemonic would be. If

echo "my mnemonic here" | ethermintd keys add mykey --recover

fails with

Error: invalid mnemonic
Usage:
  ethermintd keys add <name> [flags]

Flags:
      --account uint32           Account number for HD derivation
      --algo string              Key signing algorithm to generate keys for (default "eth_secp256k1")
      --coin-type uint32         coin type number for HD derivation (default 60)
      --dry-run                  Perform action, but don't add key to local keystore
      --hd-path string           Manual HD Path derivation (overrides BIP44 config)
  -h, --help                     help for add
      --index uint32             Address index number for HD derivation
  -i, --interactive              Interactively prompt user for BIP39 passphrase and mnemonic
      --ledger                   Store a local reference to a private key on a Ledger device
      --multisig strings         List of key names stored in keyring to construct a public legacy multisig key
      --multisig-threshold int   K out of N required signatures. For use in conjunction with --multisig (default 1)
      --no-backup                Don't print out seed phrase (if others are watching the terminal)
      --nosort                   Keys passed to --multisig are taken in the order they're supplied
      --pubkey string            Parse a public key in JSON format and saves key info to <name> file.
      --recover                  Provide seed phrase to recover existing key instead of creating

Global Flags:
  -b, --broadcast-mode string    Transaction broadcasting mode (sync|async|block) (default "sync")
      --chain-id string          Specify Chain ID for sending Tx (default "testnet")
      --fees string              Fees to pay along with transaction; eg: 10aphoton
      --from string              Name or address of private key with which to sign
      --gas-adjustment float     adjustment factor to be multiplied against the estimate returned by the tx simulation; if the gas limit is set manually this flag is ignored  (default 1)
      --gas-prices string        Gas prices to determine the transaction fee (e.g. 10aphoton)
      --home string              The application home directory (default "/Users/emmanuelodeke/.ethermintd")
      --keyring-backend string   Select keyring's backend (os|file|test) (default "os")
      --keyring-dir string       The client Keyring directory; if omitted, the default 'home' directory will be used
      --log_format string        The logging format (json|plain) (default "plain")
      --log_level string         The logging level (trace|debug|info|warn|error|fatal|panic) (default "info")
      --node string              <host>:<port> to tendermint rpc interface for this chain (default "tcp://localhost:26657")
      --output string            Output format (text|json) (default "text")
      --trace                    print out full stack trace on errors

there is no indication of what is expected and that's a less great user experience. cc @fedekunze

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