Throw error if no database URL is provided

[josephrocca]

Users may try to run code that contains repl.it db usage when they are not logged in. Currently, if they do this, they receive the following error message:

(node:83) UnhandledPromiseRejectionWarning: TypeError: Only absolute URLs are supported
    at getNodeRequestOptions (/home/runner/LimitedIncompatibleScale/node_modules/node-fetch/lib/index.js:1305:9)
    at /home/runner/LimitedIncompatibleScale/node_modules/node-fetch/lib/index.js:1410:19
    at new Promise (<anonymous>)
    at fetch (/home/runner/LimitedIncompatibleScale/node_modules/node-fetch/lib/index.js:1407:9)
    at Client.set (/home/runner/LimitedIncompatibleScale/node_modules/@replit/database/index.js:57:11)
    at /home/runner/LimitedIncompatibleScale/index.js:6:16
    at /home/runner/LimitedIncompatibleScale/index.js:9:3
    at Script.runInContext (vm.js:130:18)
    at Object.<anonymous> (/run_dir/interp.js:209:20)
    at Module._compile (internal/modules/cjs/loader.js:1137:30)
(node:83) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 3)
(node:83) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

[tekwiz]

+1

[masad-frost]

thank you, resolved in another PR.