add error handling to startup logic

[antondlr]

solves a couple issues on #238 : namely

• provide default .env file if none provided
• better error handling on startup (i.e., APIs unavailable)

additionally, the final docker image is cleaner (less layers)

[antondlr]

example output of

• BN connection not availabe
• VC connection available but wrong token provided

http://host.docker.internal:5052 unreachable, check settings and connection
validator api issue, server response: 403

[antondlr]

example output when no .env file found

No `/.env` file found at the expected location (`/app/.env`).
Please adapt this default file to your needs and mount it within the container:
----------------
PORT=3000
BACKEND_URL=http://127.0.0.1:3001
BEACON_URL=http://your-BN-ip:5052
VALIDATOR_URL=http://your-VC-ip:5062
API_TOKEN=get-it-from-'.lighthouse/validators/api-token.txt'
SESSION_PASSWORD="your-password"
SSL_ENABLED=true
----------------

[antondlr]

@chong-he
are these startup-failures verbose enough for you to assist?

[rickimoore]

LGTM