Fix Firebase example and throw meaningful errors

[penx]

Apply fixes as per comment by @mocon at #1811 (comment)

Also:

• throw meaningul errors when environment variables are missing or can't be parsed.
• check for existence of environment variable rather than cast to string

Closes: issue reported in comment #1811 (comment)

• Docs N/A
• Tests N/A

Testing Strategy:

cd examples/firebase-auth-firestore
yarn
yarn dev
# open http://localhost:3000
# assert "Error: Missing CLIENT_CONFIG environment variable" thrown as env variables are missing
# exit ^C
cp .env.example .env
yarn dev
# open http://localhost:3000
# assert "Error: Failed to parse private key" thrown as example env does not contain valid keys
# exit ^C
# manually edit .env and set invalid keys
yarn dev
# open http://localhost:3000
# assert "Error: Invalid CLIENT_CONFIG environment variable"
# exit ^C
# manually edit .env and set valid keys
yarn dev
# open http://localhost:3000
# assert login page loads

[penx]

closing as should be against dev branch