Help users figure out their ENR

[OisinKyne]

Problem to be solved

The first step on the journey to running a multi operator cluster is getting your ENR. This is a key hurdle to overcome.

Right now if you type charon enr on a fresh install this is what you get

./charon enr 
Error: load key: open .charon/data/p2pkey: no such file or directory
Usage:
  charon enr [flags]

Flags:
      --data-dir string                The directory where charon will store all its internal data (default ".charon/data")
  -h, --help                           Help for enr
      --p2p-allowlist string           Comma-separated list of CIDR subnets for allowing only certain peer connections. Example: 192.168.0.0/16 would permit connections to peers on your local network only. The default is to accept all connections.
      --p2p-bootmanifest               Enables using manifest ENRs as discv5 bootnodes. Allows skipping explicit bootnodes if key generation ceremony included correct IPs.
      --p2p-bootnode-relay             Enables using bootnodes as libp2p circuit relays. Useful if some charon nodes are not have publicly accessible.
      --p2p-bootnodes strings          Comma-separated list of discv5 bootnode URLs or ENRs. Example: enode://<hex node id>@10.3.58.6:30303?discport=30301.
      --p2p-denylist string            Comma-separated list of CIDR subnets for disallowing certain peer connections. Example: 192.168.0.0/16 would disallow connections to peers on your local network. The default is to accept all connections.
      --p2p-external-hostname string   The DNS hostname advertised by libp2p. This may be used to advertise an external DNS.
      --p2p-external-ip string         The IP address advertised by libp2p. This may be used to advertise an external IP.
      --p2p-peerdb string              Path to store a discv5 peer database. Empty default results in in-memory database.
      --p2p-tcp-address strings        Comma-separated list of listening TCP addresses (ip and port) for libP2P traffic. (default [127.0.0.1:16003])
      --p2p-udp-address string         Listening UDP address (ip and port) for discv5 discovery. (default "127.0.0.1:16004")

Error: load key: open .charon/data/p2pkey: no such file or directory

Proposed solution

Handle this error more gracefully and probably less verbosely, maybe prompt to run charon create enr in the error message.
Maybe an error like:

Error: No ENR private key found in ${data_dir}. If this is your first time running this client, create one with  `charon create enr`.

Out of Scope

If there is anything to highlight as out of scope for this issue, please outline it here.